ABO血型系统的天然抗体主要是
A．IgG
B．IgA
C．IgM
D．IgD
E．IgE
参考答案：C
He ________ his position because he had been offered a better job.
A．resigned
B．retired
C．dismissed
D．fired
风险指的是损失的( )。
A．严重程度 B．不安全性 C．不确定性 D．不稳定性
