某人上山的平均速度为3km/h，沿原路下山的平均速度为5km/h，上山用1h，则此人上下山的平均速度为（　　）

A．4km/h

B．3.75km/h

C．3.5km/h

D．4.5km/h

With so many people __________ in English every day, it will become more and more important to have a good ___________ of English.

A．communicate, knowing

B．communicated, knowledge

C．communicated, knowing

D．communicating, knowledge