先导型减压阀不起减压作用的原因可能是（）。
A.主阀在全开位卡死
B.泄油口不通
C.先导阀打不开
D.A或B或C
参考答案：D
According to the statement, the UN Security Council resolution concerning Israel and Yasser Arafat was presented by the United States and approved by 11 Council members including Britain, Germany and Bulgaria.
检查椎间盘和脊髓的最佳方法是（）
A.MRI
B.CT
C.ECT
D.X线平片
E.超声成像
