Answer:
A) sea level drops; land subsides
Explanation:
The sea is the final destination of all streams on land. All streams empty into seas which are the basins that collects all terrestrial materials.
The base level is the lowest level to which a river can erode its channel. The base level is greatly influenced by what happens to a sea.
When a river reaches its base level, erosion ceases. If a river is young and set above its base level, we see a rapid rate of erosion.
A sea, the final basin of depostion of rivers controls the base level to a very large extent. When there is a tectonic movement and a sea subsides. It becomes more deeper and lower. Sea level also falls and materials grades seaward. A river would cut rapidly along its channel and its bed to measure up to the level of the ocean.
Therefore, we see a very rapid rate of erosion of channels when sea level falls and the earth sags downward. This would lead to a lower base level.
When sea level rises and there is a tectonic uplift, rivers erode their channels less and begins to aggrade materials. The base level also rises.
