—Where are the pens?
—______ on the desk. [ ]
A. They're
B. It's
C. This is
答案:A
产蛋鸡日粮钙含量应保持().
A.1%左右
B.5-6%
C.3-4%
D.10-20%
极坐标法适用于测设点离控制点较近且便于量距的情况。( )