There is plenty of rain in the southeast,         there is little in the northwest.

A．because

B．therefore

C．while

D．as

内燃机车励磁电路中，电阻（）的整定值决定了同步牵引发电机的最高电压限制。

A.Rgt

B.R1cf1

C.R1cf2

D.R1cf3