Discovery of ridges and trenches on the ocean floor weakened which idea?

Correct answer: The ocean floor is completely flat and inactive

Direct answer: Option A, the idea that the ocean floor is completely flat and inactive. Ocean-floor ridges are long elevated underwater features, while trenches are deep narrow depressions. Their discovery showed that the sea floor has varied relief and is connected with geological activity. This evidence weakened the old oversimplified belief that the ocean floor was an unchanging, level surface. Option A is correct because ridges and trenches directly contradict “completely flat and inactive.” Option B is wrong because the presence of water over the ocean floor is an obvious condition, not an idea weakened by discovering landforms beneath it. Option C is wrong because rocks on continents are unrelated to the specific conclusion about ocean-floor relief. Option D is wrong because gravity is not challenged by the discovery of ridges and trenches. Reasoning: underwater features were observed; therefore the floor is not uniformly flat; geological processes must be considered. Memory cue: ridges rise and trenches sink, so the sea floor is not a flat sheet.