They/ not answer/phone/ this morning, so/ must/ out/. They hasn't answered
Giải thích
Chọn C
A. They hasn't answered the phone this morning so they must have been out. (sai thì giữa 2 vế)
B. They didn't answer the phone this morning so they must be out. ( sai vì không đồng nhất thì giữa 2 vế)
C. They didn't answer the phone this morning so they must have gone out.
D. They hasn't answered the phone this morning so they must go out. (sai thì giữa 2 vế)
ð Đáp án C