_____youunderstandthisrule，you‘llhavenodifficultyindealingwithsuchproblems.
D
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。
_____alltherepresentativesstillnotthere，themeetingwaspostponedforseveralhours.
Thegirlwhofoundtheringreceivedagenerous_____offivepounds.
Itseemsasifthesun_____roundtheearthsinceitrisesintheeastandsetsinthewest.