一次负荷的安全性是指X线管的()
A.容量
B.寿命
C.热容量
D.散热率
E.最高管电压
参考答案:A
解析:容量是X线管在安全使用条件下能承受的最大负荷量。
照相机是根据凸透镜能成________像的光学原理制成的.当拍摄近的景物时,镜头要往前冲,离胶片_______些.光圈是用来控制进入镜头的_______多少的;快门是用来控制曝光_________的.
Sorry, I am too busy now. If I________ time, I would certainly go for an outing with you.[ ]
A. have had
B. had had
C. have
D. had