_____you understand this rule ，you‘ll have no difficulty in dealing with such problems.
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。
Williamhascuthissmoking_____tofivecigarettesaday.
I‘vebeentellingyou_____nottodothat，butyounevelistenedtome.That’swhyyougotintotrouble.
Hedrovefastandarrivedanhour_____ofschedule.
Theteachergave_____theexaminationpaper.
_____youunderstandthisrule，you‘llhavenodifficultyindealingwithsuchproblems.
