Your son has kept _________ for two hours. You’d better ask him to have a rest.

A．study

B．studied

C．to study

D． studying

产品策略是销售工作的一个重要内容，通常采用的产品策略有()。

A.厚利限销

B.不断创新

C.提高产品质量

D.迅速及时

E.低本价廉