What's the matter,Ali? You look sad . Oh , nothing much . A a matter of fact, I __my friends back home .
A:just thought
B: have just been thingking
C:was just thingking
D:have just thought
____four years since John left school.
A: They have been
B: It is
C: It was
D: Those are
The President ____ the delegation at the airport but he wan taken ill last night, so the vice-president is going instead.
A: is to meet
B:was to meet
C:has been to meet
D:was to have met
答案是:C B B
为什么?给个详细的注解先