把下面各数的分解质因数．
49579196．
49=7×7；
57=3×19；
91=7×13；
96=2×2×2×2×2×3；
It is hard to catch those who _______ the small advertisements at night. [ ]
A. put up
B. put off
C. put on
D. put away
排水管道堵塞的原因有（）。
A.管道敷设坡度太小或有倒坡现象
B.水中杂质在管内沉淀下来
C.排水量过大
