东、中、西三大地带的关系是( )地，区经济结构。
A．第一层B．第三层C．第二层D．第四层
参考答案：C
What have we said _______ her so happy?
A．makes
B．to make
C．made
D．has made
After each reading of an oxygen indicator, the instrument should be purged with（）.
A.CO2
B.fresh air
C.the tested compartment's air
D.water
