题目

—Have you ________ been to our town before? —No, it's the first time I ________ here.A.ever; have come B.never; come C.ever; came 答案：【答案】A【解析】句意：——你以前来过我们镇吗？——不，这是第一次我来这。考查副词及动词时态。ever曾经；never从不。根据“Have you … been to our town before”可知，询问是否曾来过我们镇，所以第一个空填ever。根据问句“have”可知，答句应用现在完成时，故选A。20．为胚胎发育提供主要营养物质是卵黄．