烧结空心砖的主要原料有（）
A.黏土
B.电石膏
C.煤矸石
D.砂
E.页岩
参考答案：A, C, E
Do ________ you think is right _________ difficulties you may have.
A．what; however
B．that; whatever
C．whatever; however
D．what; whatever
※ [18～20] .( 3 )
A． .
B． .
C． .
D． .
