There are three on the desk.
A:pieces of papers
B:piece of papers
C:pieces of paper
沙子的主要成分是SiO2,SiO2中硅元素的化合价是( )
How many girls?
如图为直流电源电解稀Na2SO4水溶液的装置.通电后在石墨电极a和b附近分别滴加一滴石蕊试液,下列实验现象中正确的是( )