安全阀密封面采用D507焊条堆焊，哪个说法是正确的（）
A、必须采用直流焊机
B、必须采用反极性接法
C、焊条施焊前应烘干
D、以上全是
参考答案：D
John was honest _____ his companions and won their faith _____ him. [ ]
A. to; for
B. with; in
C. for; about
D. with; of
静脉肾盂造影检查的禁忌证是：()
A．肾盂积水
B．肾结石
C．尿路狭窄
D．肾部肿瘤
E．肾功能严重损伤
