放射性的强度是用什么来度量的（）
A.尺寸大小
B.活度
C.源的种类
D.能量
参考答案：B
工业用盐亚硝酸钠（NaNO2）外观极像食盐，人若误食亚硝酸钠会中毒．亚硝酸钠中氮元素的化合价是（ ）
A．+l
B．+2
C．+3
D．+4
was born being good at all things. You can do well in things through hard work. As an English learner, you can’t be excellent without enough practice.
A. Everybody B. Somebody C. Nobody
