X线管的焦点包括
A.实际焦点
B.有效焦点
C.大焦点
D.小焦点
E.虚焦点
参考答案:A,B,C,D
解析:X线管的焦点包括实际焦点、有效焦点、大焦点、小焦点。
已知圆C:x2+y2=1,点P(x 0,y0)在直线x-y-2=0上,O为坐标原点,若圆C上存在点Q,使∠OPQ=30°,则x0的取值范围是 ( )
A.[-1,1]
B.[0,1]
C.[-2,2]
D.[0,2]
His name is James but he calls ________ Jim.[ ]
A. his
B. himself
C. him
D. 不填