9) D ( cấu trúc: It's essential + to V = should have + Ved )
10) C ( It's better for smb to do smt = It's high time + S + quá khứ )
11) D ( cấu trúc càng j đó thì càng j đó : The less + (long )adj - the + (short )adj-er )
12) A ( Despite+ V-ing/N )
13) D ( Cấu trúc mặc dù, nhưng vẫn : Adj as S + be, S + V )