The manager listened to the customers’ complaints attentively with great patience, _______ to miss any point.

A．not trying

B．trying not

C．to try not

D．not to try

获得守恒性和可逆性概念是在哪一阶段（）

A.感知运算阶段

B.前运算阶段

C.具体运算阶段

D.形式运算阶段