题干
—Which is more useful, reading or writing?
—I think _______ of them are useful.
A：either
B：both
C：all
D：each
答案(点此获取答案解析）
B
—Which is more useful, reading or writing?
—I think _______ of them are useful.
A：either
B：both
C：all
D：each
的展开式中的常数项是（ ）