We laugh at jokes, but seldom _____ about how they work.  [ ]

A. we think

B. think we

C. we do think

D. do we think

设A为己定义的类名，则下列声明A类的对象a的语句中正确的是( )。

A．public A a=new A()；

B．public A a=A()；

C．A a=new class()；

D．a A：