题干
-----What were you up to when she dropped in?
------I______ for a while and _____ some reading.
------I______ for a while and _____ some reading.
A：had played ; did
B：played; did
C：had played :was going to do
D：was playing; was going to do
答案(点此获取答案解析）
C
∈Q (2）0
N (3）
{1，2} (4)φ={0}