You will find the game is not so difficult to play you understand its rules.
A. until
B. since
C. before
D. once
网友回答
A. until
B. since
C. before
(答案→)D. once
解析：考查状语从句。句意：一旦理解了比赛规则，你就会发现这个比赛并不难。A直到；B自从；C在......之前；D一旦。选A的话，意为“直到理解了规则才发现它很难”，不符合题意。故选D。