descend 通常指沿斜线或斜坡下降。
drop 指物体从一定高度落下。
fall 与drop同义，指突然或猛烈地降落，但fall也可指任何下落，同高度或形式无关。
sink 指在空气或水中垂直下降、下沉。
参考例句
1. Our plane sank to the height of 3 kilometres. 我们的飞机下降到了三千米的高度。
2. Unblocked the sink in the Canteen. 餐厅的洗涤槽不阻塞了。
3. I shall plunge into the pool although but to sink to the bottom. 我一定要条近池去，尽管我跳进去也不过是沉到水底。
英英解释
n.
1. plumbing fixture consisting of a water basin fixed to a wall or floor and having a drainpipe
2. (technology) a process that acts to absorb or remove energy or a substance from a system
3. a depression in the ground communicating with a subterranean passage (especially in limestone) and f
4. a covered cistern; waste water and sewage flow into it v.
1. fall or drop to a lower place or level
2. cause to sink
3. pass into a specified state or condition
4. go under,
5. descend into or as if into some soft substance or place
6. appear to move downward
7. fall heavily or suddenly; decline markedly
8. fall or sink heavily
9. embed deeply
