两电容器的电容之比为C1:C2=1:2。(1)把它们串联后接到电压一定的电源上充电,它们的电能之比是,(2)如果是并联充电,电能之比是
A. 2:1,2:1
B. 1:2,2:1
C. 2:1,1:2
D. 1:2,1:2
一平行板电容器,两板相距为d,与一电池联接时,相互作用力为f。若将电池断开,把距离拉到3d,则相互作用力变为:( )。
A. f/3;
B. 3f;
C. 9/f;
D. 不变。
Maths, one of the most important subjects, ______always interested him.
A. has
B. have
C. are
D. is
I’m Chinese. Where______from?
A. do you come
B. you are coming
C. you come
D. are you coming