题干
—I called you at 4:00 yesterday afternoon, but no one answered.
—Sorry, I ______ with my friends at that time.
—Sorry, I ______ with my friends at that time.
A：swim
B：swam
C：will swim
D：was swimming
答案(点此获取答案解析）
D
( )
,a
,a
,a
,a
}的真子集个数是（ ）