Mapping of the ocean floor

Correct answer: A. Because it measures the return time of sound waves

Explanation: The governing concept is sonar-based bathymetry, the measurement and mapping of underwater depth. A sonar instrument sends a sound pulse downward through the water. The pulse reflects from the sea floor and returns to the receiver. If the return time and the speed of sound in water are known, the approximate depth can be calculated using distance = speed × time, with the result divided by two because the pulse travels down and back. Repeated measurements along routes reveal variations in depth and help construct a map of the ocean floor. Thus A is correct; sonar does not dry water, measure only clouds, or remove salt.

Correct answer: A. Sonar

Explanation: The correct answer is A, sonar. Sonar means sound navigation and ranging. It sends sound pulses through water and measures the time taken for echoes to return from the seabed. Using the travel time and the known or estimated speed of sound in seawater, scientists calculate depth and identify underwater relief such as ridges, trenches, and channels. A thermometer measures temperature, a barometer measures atmospheric pressure, and a rain gauge measures precipitation. Bathymetry is the broader measurement and description of underwater depth and terrain; sonar is one important technique used to collect bathymetric information. Hence the sound-wave method is sonar.

Correct answer: A. Bathymetry

Explanation: Bathymetry is the measurement and systematic mapping of water depth and the shape of the underwater terrain. Navigation charts use bathymetric data to show depth contours, channels, shoals, ridges, trenches, and other hazards that may affect the safe passage of ships and submarines. Depth is commonly measured using sonar and related surveying techniques, after which the observations are converted into charts or digital seabed models. Crop density, rainfall amount, and soil colour describe land or atmospheric conditions and cannot indicate the depth of seawater. Therefore, bathymetry is the precise term required by the question. It is also an important method for studying ocean-floor relief.

Correct answer: A. Places of equal depth

Explanation: The governing concept is bathymetric mapping. Lines drawn on a map to join points having the same water depth are called bathymetric contours, or isobaths. They perform a role similar to contour lines on a landform map, but instead of showing equal elevation above sea level, they show equal depth below the sea surface or a chosen datum. Closely spaced lines indicate a rapid change in depth and therefore a steep underwater slope; widely spaced lines suggest a gentler slope. Option A is correct because it states the exact meaning of these lines. Options B, C, and D describe temperature, rainfall, and population patterns, which require different thematic lines and are unrelated to bathymetry.

Correct answer: A. Both show differences in elevation or depth

Explanation: The governing concept is relief mapping. A normal topographic map represents the shape of land by showing differences in elevation above a reference level, often with contours or related symbols. A bathymetric map applies the same basic idea below water: it represents the shape of the sea floor by showing variations in depth, also commonly through contours, colour bands, or depth values. Thus option A correctly identifies their similarity. The two maps differ mainly in the surface they depict—land in one case and submerged terrain in the other—not in the fundamental purpose of representing relief. Options B, C, and D describe thematic maps rather than the defining similarity of these maps.

Correct answer: A. Because it helps estimate distance to the sea floor

Explanation: The governing concept is echo sounding, or sonar-based measurement of underwater depth. A sonar instrument sends a sound pulse toward the sea floor and records the time taken for the reflected echo to return. If the speed of sound in the water is known or appropriately estimated, the total travel distance is calculated as speed multiplied by time; because the pulse travels downward and upward, the depth is approximately half of that total distance. Therefore, option A is correct. The return time provides the distance information. Options B, C, and D describe effects unrelated to the physical principle of sonar.

Correct answer: A. Bathymetry is depth study and sonar can be a major tool for it

Explanation: Bathymetry is the measurement and description of underwater depth and the shape of the seafloor. Sonar, especially echo sounding or multibeam sonar, is a principal technology used to collect these data: it sends sound pulses through water, records the returning echoes and converts travel time into depth after accounting for sound speed. Repeated measurements from a vessel can then be compiled into bathymetric charts or digital relief models. Thus option A correctly distinguishes the subject of study from a useful measuring tool. Sonar does not measure rainfall, and bathymetry does not study clouds. Neither technique is limited to coastal population, while option D incorrectly denies their central relationship with ocean-floor mapping.