单项选择题

One must have studied hard before one could succeed in mastering a foreign language.()

A.在掌握一门外语之前只有刻苦学习。
B.不下工夫是掌握不了外语的。
C.谁肯下工夫,谁就能精通外语。
D.必须勤学苦练,才能精通一门外语。