下列因素中可增加对比度的因素是（）
A.过度显影
B.散射线多
C.本底灰雾大
D.被检体太厚
E.使用滤线器
参考答案：E
He _____ on the bench, with his head down and his eyes _____ on the ground.
A．was seated; fixing
B．was seated; fixed
C．seated; fixing
D．seating; fixed
It's very cold today. Please _______ the door. [ ]
A. open
B. don't open
C. not open
D. did't open
