用配方法将二次三项式a2-4a+5变形，结果为( )
A：(a-2)2+1
B：(a+2)2+1
C：(a-2)2-1
D：(a+2)2-1
已知∠AOB=50°，∠BOC=30°，则∠AOC=____．
It ____for them to offer you the chance.
