We are living in an age _____ many things are done on computer.

A．which

B．that

C．who

D．when

皮亚杰认知发展的阶段理论认为，守恒观念出现的关键是( )

A．多维思维

B．自我中心思维

C．可逆思维

D．具体逻辑思维