After the Arab states won independence, great emphasis was laid on expanding education, with girls as well as boys _____ to go to school.

A．to be encouraged

B．been encouraged

C．encouraged

D．be encouraged

变压运行时机组负荷变化速度快慢的关键在于（）。

A、锅炉燃烧的调整；

B、汽轮机调门的控制；

C、发电机的适应性；

D、蒸汽参数的高低。