建筑石膏是一种以（）为主要成分的气硬性胶凝材料。
A.硫酸镁
B.硫酸亚铁
C.硫酸钙
D.硫酸铁
参考答案：C
Who's this girl?[ ]
A. He's my uncle.
B. She's my sister.
We are taught that a business letter should be written in a formal style ________ in a personal style. [ ]
A. rather than
B. other than
C. better than
D. more than
