20%=8÷40=0.2=25×12=二成；
故答案为：8，0.2，12，二．
President Obama ____ a visit ____ China last month. [ ]
A. paid, to
B. gave, in
C. paid, for
单次覆盖观测系统常见基本类型有（）种。
A.2
B.3
C.4
D.5
