6502电气集中系统在最不利的条件下，应该满足（）原则。
参考答案：故障—安全
—How long does it take us to go to your hometown from here?
—It takes us ______ hour or more to go to my hometown by ______ train.[ ]
A. an; a
B. a; an
C. an; /
D. a; /
箱式变电所的占地面积一般为（）m2。
A.5-6
B.6-8
C.8-10
