A:HCl
B:NaOH
C:Na2SO4
D:NaCl
Dear students, please read every sentence carefully. _______ you are, _______ mistakes you'll make.
Mr. Smith arrived at the station______.
出现成语“南橘北枳”现象的原因是( )
据有关资料介绍,儿童缺钙会得佝偻病,成年人缺钙会得软骨病,血液中也有少量钙,它对皮肤伤口血液的凝固起着重要作用;人体缺碘会引起甲状腺肿大。这里的“钙”、“碘”是指 ( )
某青蛙饲养场,为缩短蝌蚪发育为成蛙的周期,常用含某种激素的饲料给蝌蚪喂食.这里所说的“某种激素”应该是()