8, A -> were (tobe trong mệnh đề if ở câu điều kiện loại 2 luôn là were)
9, B -> polluting (stop+Ving: dừng việc gì)
10, B -> occurred (thì HTHT)
11, B -> went
12, C -> wont wait
13, C -> took
14, C -> so
15, A -> had memorized
16, B -> having
17, B -> waiting
18, A -> to visit
19, A -> will be
20, A -> possible