— Look! How long ________ like this?

— Three weeks! It's usual here that rain ________ without stopping these days of the year.[ ]

A. has it rained; poured      

B. has it been raining; pours

C. is it raining; is pouring    

D. does it rain; has poured

如图所示,平行导轨间的距离为d.一端跨接一个电阻R,匀强磁场的磁感应强度为B,方向垂直于平行金属导轨所在平面向里.一根足够长的金属棒与导轨成θ角放置,金属棒与导轨的电阻不计,当金属棒沿垂直于棒的方向滑行时,则ab的电势差为          ，   .(填写“﹤”“﹥”“﹦”)