电动机装接地线主要是避免（）事故的发生；所以，（）应检查接地线是否良好。
参考答案：人身触电；启动前。
A number of books ______ been stolen from the library since the beginning of this term and the number _____ as large as 500.
A．have; are
B．has; is
C．has; are
D．have; is
装置空速的选择，一般要考虑到（）。
A、装置投资
B、催化剂的活性
C、原料性质
D、产品要求
