Completing all the homework by the dinner, John felt very tired. He __________all day long.

A．was studying

B．studies

C．has studied

D．had been studying

在面向对象程序设计中，对象信息的隐藏主要是通过( )来实现的。

A．对象的封装性

B．子类的继承性

C．系统模块化

D．模块的可重用性