题干
I usually go to school by bike， today I took a bus because it rained heavily．
A：but
B：so
C：if
D：or
答案(点此获取答案解析）
A
I usually go to school by bike， today I took a bus because it rained heavily．
A：but
B：so
C：if
D：or
变形为 
，则m+n=