如图所示，一段长方体形导电材料，左右两端面的边长都为a和b，内有带电量为q的某种自由运动电荷．导电材料置于方向垂直于其前表面向里的匀强磁场中，内部磁感应强度大小为B．当通以从左到右的稳恒电流I时，测得导电材料上、下表面之间的电压为U，且上表面的电势比下表面的低．由此可得该导电材料单位体积内自由运动电荷数及自由运动电荷的正负分别为（   ）

I

⑴What's that in English?   ____

⑵What color is the pen?   ____

⑶How are you?   ____

⑷Who's the girl?   ____

⑸He's my brother, Jack．   ____

⑹Is he your friend?   ____

⑺Good evening, Bob!   ____

⑻Where's your key?   ____

⑼Are these Dale's cards?   ____

⑽Have a good day!   ____

II

A．Yes, they are．

B．I'm fine, thanks

C．On the book．

D．It's a ruler．

E．Good evening!

F．She is Kate．

G．No, he isn't．

H．It's black．

I．You, too．

J．Oh, I see.