16.D. Please tell me the reason for which you cry.
for which=why
17.A. That is the house on which my teacher is living now.
on which=where
18.A. Where are you working now?
Hiện tại tiếp diễn
19.D. What was she doing when you saw her?
Quá khứ tiếp diễn
20.C. How many students are there in your class?
students số nhiều đếm được nên dùng many và are