竹笼围堰一般使用的年限为( )年。
A．1～2
B．3～4
C．1～3
D．2～5
参考答案：A
—________?
—Father's Day is coming. I'd like to buy a present for my father. [ ]
A. How much would you like
B. What can I do for you
C. What colour do you want
D. What size would you like?
（10分）如图所示电路中，U=10V，R1=4Ω，R2=6Ω，C=30μF，电池的内阻可忽略．求：
(1)闭合开关S，求稳定后通过R1的电流；
(2)将开关断开，这以后流过R1的总电量。
