【题目】You won't find it difficult ______ you have mastered it.
A.before
B.while
C.once
D.though

【答案】C
【解析】句意：一旦你掌握了它，你就不会觉得它很难了。A. before在……之前；B. while当……的时候；C. once一旦；D. though尽管。根据句意可知选C。