Americans _________sent a message to the world________ we have never been a collection of Red States and Blue States: we are, and always will be, the United States of America.               

A．that, \

B．who, that

C．who, when

D．that, what

雷电过电压计算时，风速应为( )。

A．10m/s
B．15m/s
C．20m/s
D．29m/s