The children ＿＿very quiet；I wonder what they ＿＿up to.A. were； are beingB. are being；areC.

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。