—How many times have you been to China?

—In fact, it is the first time I __________ here.

A．have been

B．am

C．had been

D．was

在普通V带传动中，载荷平稳，包角为180º，带长为特定长度，单根V带的基本额定功率P。主要与（）、（）和小带轮转速有关。