切换流程操作均应遵循（）的原则。
A.先关后开
B.同时关
C.同时开
D.先开后关
参考答案：D
With Chang'e-3 successfully landing, China becomes the third country in the world that _______ on the moon.
A．lands
B．has landed
C．landed
D．was landing
三才升
