对比度是指钻石内含物的（）同钻石体色之间的对比程度。
A.颜色
B.光泽
C.透明度
D.A+B+C
参考答案：D
He had to pause from time to time to wipe the sweat from his forehead, because the
air-conditioning system ____.[ ]
A. broke in
B. broke up
C. broke out
D. broke down
并联电路中每个元件所承受的（）是相等的。
A.电流
B.电阻
C.电导
D.电压
