在粉煤灰化学成分中，（）约占45%—60%。
A.Al2O3
B.Fe2O3
C.SiO2
D.CAO
参考答案：C
Aunt Liu ______two sons. They ______ students. [ ]
A. have, are
B. has, is
C. has,are
外加剂抗压强度比结果应精至（）%
A.1%
B.2%
C.5%
D.3%
