地线应满足电气和机械使用条件要求,可选用( )之一。
A.钢芯铝绞线;
B.钢芯铝合金绞线;
C.镀锌钢绞线;
D.复合型绞线。
参考答案:C,D
He _________ his bicycle when it began to rain.[ ]
A. was riding
B. is riding
C. has ridden
D. rides
只有通过操作系统才能完成对计算机的各种操作。( )