生物课上,老师举办了一个“我来提问你来答”的比赛,并将比赛分为4轮,答对老师提出的问题的同学就可以晋级,进行下一轮比赛,答错的则淘汰.比赛时,老师提出了这样一个问题:叶肉栅栏组织的特点是什么?下面四位同学中可以晋级的是( )
A:晓静:细胞呈椭圆形,排列较整齐
B:晓梅:细胞呈圆柱形,排列较整齐
C:晓宇:细胞呈长方形,排列较疏松
D:晓雯:细胞呈正方形,排列较疏松
秦国在正式变法以前,秦孝公主持会议就是否变法进行争论,商鞅针对当时守旧势力的观点进行了针锋相对的辩驳,下列观点属于他的是( )
某同学被蛇咬伤后,医生为他静脉注射抗毒素血清,这种免疫方式及抗毒素血清分别属于( )
“Early to bed, early to rise(起身)makes people healthy, wealthy and wise(健康,富有,聪明).” This is an old English saying. Children should have ten hours' sleep every night, or they can't do their work very well. They will not be wise. The body needs exercise. Walking, running, swimming and playing ball games are all exercise. Exercise keeps the body strong.
如图所示,ΔABC中,∠BAC=90°,AD⊥BC于D,若AB=3,BC=5,则AD的长度是( )