Alxander Ferguson was a worker on shipyards ________ he was 23, ________ he began to play professional football．A.when; whenB.until; whenC.when; whichD.where; when
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B解析考查连词。句译为：Alxander Ferguson直到23岁还是一名船厂工人，(但)这时他开始专业踢球了。until“直到……时候才(不)……”；when“当……时候”。