形成叶片的阻尼与衰减大体有三类，分别为（）、（）和（）三类。
参考答案：材料，叶片榫头，空气衰减
If the weather had been better, we could have had a picnic. But it ______ all day. [ ]
A. rained
B. rains
C. has rained
D. is raining
输尿管结石X线平片可见（）
A.呈卵圆形致密阴影
B.结石边缘多毛糙不整齐
C.其长轴与输尿管走行一致
D.结石常见于输尿管三个生理狭窄处
E.梗阻上方输尿管及肾盂肾盏不同程度扩张积水
