_____youunderstandthisrule，you‘llhavenodifficultyindealingwithsuchproblems.
A、WhileD
once 意思是 “一旦 ”。while “ 当⋯⋯ 时候 ” ;though“虽然”;unless “除非 ” 。“一旦 你明白了这个规则，处理类似的问题就不困难了。 ”显然， d 符合句子意思的表达需要。
It seems as if the sun_____round the earth since it rises in the east and sets in the west.
Children who are over-protected by their parents may become_____.
Your bike needs_____.You ‘d better do i right now.
He drove fast and arrived an hour_____of schedule.
Williamhascuthissmoking_____tofivecigarettesaday.
