如图所示，物体A贴在竖直墙面上，在竖直轻弹簧作用下，A、B保持静止。则物体A的受力个数为
A．2
B．3
C．4
D．5
答案：B
Look!One of the children _____ in the river.
A．was swimming
B．is swimming
C．are swimming
D．were swimming
活性污泥净化污水在第一阶段的吸附作用进行得十分迅速，一般在（）完成。
A.10min
B.15min
C.30min
D.60min
