Why does vegetation change step by step from lower to upper parts of high mountains?

Correct answer: Temperature, moisture and soil conditions change with altitude

The direct answer is option A: temperature, moisture, and soil conditions change with altitude. A mountain is not climatically identical from its base to its summit. As height increases, air temperature generally falls, moisture and precipitation patterns may change, and soils become thinner or different. These changes create vertical vegetation belts: plants suitable for warm lower slopes may not survive in colder upper areas. Option A is correct because it gives the connected physical causes. Option B is wrong because plants do not become the same; they become different according to conditions. Option C is wrong because altitude strongly affects climate, especially temperature. Option D is wrong because mountains receive sunlight, although clouds, slope, and season can alter its amount. The exact vegetation pattern can vary by slope and region, but the principle of altitude-related change remains. Memory cue: going upward means temperature usually goes downward, so vegetation changes in belts.