I had wanted to help you last night but I couldn't spare any time, for I ______ a composition which

I have to hand in this morning. [ ]

A. wrote

B. was writing

C. had written

D. have written

线圈中感应电动势大小与（）

（A）线圈中磁通的大小成正比，还与线圈的匝数成正比

（B）线圈中磁通的变化量成正比，还与线圈的匝数成正比

（C）线圈中磁通的变化率成正比，还与线圈的匝数成反比

（D）线圈中磁通的变化率成正比，还与线圈的匝数成正比