一个最简分数,分子和分母的和是15,这样的分数一共有( )个。
7
高kV摄影设备的叙述,错误的是()
A.用高栅比滤线器
B.常用栅比为12:1
C.γ值高的X线胶片
D.120~150kV管电压的X线机
E.阳极为散热性好的铜体上嵌钼
I’m sure that if you make good use of your ____ time, your English will be improved in a short time.
A.extra
B.common
C.special
D.spare