问题补充:
The children ______ very quiet; I wonder what they ____up to.
A. were; are being
B. are being; are
C. are; do
D. are being; do
答案:
A. were; are being
(答案→)B. are being; are
C. are; do
D. are being; do
解析:be用于进行时可表示一个暂时的特点或表现。What they are up to相当于What they are doing。