题目

I didn’t accept the job, not that I am unwilling to do it, ______ that I am unable to.   A. so   B. but    C. however    D. though   答案：B3．羊x头，牛的头数比羊的3倍少25头，牛：（3x-25）；牛x头，牛的头数比羊的3倍少25头，羊：（x+25）÷3．