滤线栅摄影中照片影像中心呈现一定密度，两侧无密度原因可能是（）
A.滤线栅反置
B.侧向倾斜
C.上栅焦距偏离
D.双重偏离
E.下栅焦距偏离
参考答案：A
I _________ an old photo while __________ my album, which reminded me of my happy childhood.
A．came across, looking through
B．came about, looking up
C．came along, looking down upon
D．came up with, looking into
普通水泥砂浆麻杆其砂浆标号不低于C20。（）
