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。
