有些中学生在学习抽象概念和规则时，仍需具体经验支持，从认知发展阶段理论来看这是因为 ( )

A．这些概念、规则太复杂

B．有利于他们进行发现式学习

C．他们尚未达到形式运算阶段

D．具体经验可以帮助他们加深印象

He ______ so much work that he couldn’t really do it efficiently.

A. put on
B. brought on
C. took on

D. carried on