题干
It was very of you to let us know you were going to be late
A：suitable
B：considerate
C：worthwhile
D：modest
答案(点此获取答案解析）
B
中， 
， 
，将 
绕点O沿逆时针方向旋转 
得到 
，连结 
，求证：四边形 
是平行四边形．