志留纪分(),石炭纪分二世,二叠系分三统,白垩系分二统。
参考答案:四世
—What are you doing for your winter vacation?
—I _____ my aunt in Kaifeng. [ ]
A. visit
B. am visiting
C. visited
D. visits
脑膜瘤术前栓塞最常用的栓塞物质为()。
A.明胶海绵颗粒
B.碘油
C.微粒
D.无水酒精
E.自身血凝块