R1与R2并联，如果R1=100Ω，R2=300Ω，并联后的电阻是（）Ω。
A.75
B.100
C.300
D.400
参考答案：A
A．It may have a lot of fun.
B．It’s a good opportunity for people to know the society.
C．It’s one of the most difficult jobs.
D．It’s a challenge for people’s ability.
钢芯铝绞线中的钢丝，一般钢丝的疲劳极限为（）kgf/m㎡。
A.10～15
B.16～19
C.30～36
D.35～40
