Although he          in America for a long time, he’s never regretted his making his mind to return to his own country.

A．stayed

B．was staying

C．has stayed

D．had stayed

在互换性问题中，( )为初期所使用的一个互换性判定指数。

A．引射系数
B．核准基数
C．比例常数
D．华白数