题干
. It is necessary for us to learn _____ second language ____ our mother tongue.
A：the; beside
B：a; except
C：the; except
D：a; beside
答案(点此获取答案解析）
D
A：the; beside
B：a; except
C：the; except
D：a; beside
8SO2+2Fe2O3，其中FeS2是（ ）