_____you understand this rule ，you‘ll have no difficulty in dealing with such problems.
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。
Ifhe_____thestorekeeper‘sscissors，hewouldhaveforgottentobuyapair.
Beingextremely_____tothecold，Idonotlikeskating.
_____alltherepresentativesstillnotthere，themeetingwaspostponedforseveralhours.
Thegirlwhofoundtheringreceivedagenerous_____offivepounds.