2甲基1,3丁二烯和足量的溴发生加成反应，其加成产物中二溴代烃有( ) A．1种 B．2种 C．3种 D．4
如表所示某生态系统内各相关物种之间食物的比例关系。回答下列有关问题： 草植食性鸟兔子老鼠虫青蛙蛇鹰 植食性鸟100% 兔子100% 老鼠100% 虫100% 青蛙
-What ______ do you have for Paul? -I think he should study harder than before. A．news B．advice C．help D．information
根据表中的数据,从电负性的角度判断下列元素之间易形成共价键的一组是( ) 元素NaMgAlHCOCl 电负性0.91.21.52.12.53.53.0 ①Na和Cl ②Mg和Cl ③Al和Cl ④H和O ⑤Al
.【2011·皖南八校第三次联考】B If a business wants to sell its products internationally, it had better do some market research first．This is a lesson that some large American corporati
若顺次连结四边形各边中点所得的四边形是菱形，则原四边形（ ） A．一定是矩形 B．一定是菱形
Our teachers won’t allow to cheat in the examinations.
计算 tan2°tan4°・tan6°…tan88°
= 
C．1 D．
