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
网友回答
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。