They were surprised that a four-year-old boy should work out such a difficult problem ____ they themselves couldn’t. A. once B. then C. while D. if 答案:Cwhile是并列连词,意为“而,却”,前后有对比的意味。读某区域400米以下的岩层剖面局部图,回答3~4题。 3.从图中的岩层剖面图看,该区域最有可能是( ) A.背斜 B.向斜 C.地垒 D.地堑 4.上图中甲岩石最有可能是( ) A.玄武岩 B.页岩 C.板岩 D.大理岩