绝缘材料在运行过程中受热、光、氧等因素长期作用而逐渐失去绝缘性能的现象称为()。
A、击穿
B、老化
C、外因损伤
参考答案:B
He is the man _________ you can turn when you are in trouble.
A.that
B.which
C.to whom
D.to him
组成天然蛋白质的氨基酸有()种。
A.10
B.20
C.30
D.40