题干
Learning a foreign language is especially difficult for those who have never learned ______before.
A：one
B：it
C：them
D：that
答案(点此获取答案解析）
B
Learning a foreign language is especially difficult for those who have never learned ______before.
A：one
B：it
C：them
D：that
），则f（ 
）的值为（ ）