低毒类的有机磷杀虫药是( )
A．马拉硫磷
B．甲拌磷(3911)
C．对硫磷(1605)
D．内吸磷(1059)
E．敌敌畏
参考答案：A
缺血型视网膜中央静脉阻塞可能出现（）
A.闪光ERG检查a波正常，b波下降
B.闪光ERG检查a波下降，b波正常
C.闪光ERG检查a波b波均下降
D.闪光ERG检查Ops波下降
E.闪光ERG检查视锥细胞反应异常，视杆细胞反应正常
Xavier, Y vonne, and Zelda each try independently to solve a problem. If their individual probabilities for success are and , respectively, what is the probability that Xavier and Yvonne, but not Zelda, will solve the problem ？（）
A.A
B.B
C.C
D.D
E.E
and 
, respectively, what is the probability that Xavier and Yvonne, but not Zelda, will solve the problem ？（）