题型:填空题 氢是新型清洁能源,但难储运,研究发现,镧和镍的一种合金是储氢的良好材料.其中的镧(La)元素核电荷数为57,相对原子质量为139,则下列关于镧元素的说法正确的是( )A.原子内有139个质子B.原子核内有57个中子C.原子核外电子数为82D.镧元素为金属元素 查看答案
题型:填空题 If law and order as well as rules and regulations _____, neither the citizen nor his property is safe.[ ]A. are not observed B. is not observed C. were not observedD. was not observed 查看答案