A:NaOH固体
B:CH3COONa固体
C:NaCl晶体
D:Na2CO3固体
用代入法解方程组2x+3y=75x-y=9先消去未知数( )最简便.
如图所示,做有关声现象实验时,小华将发声的音叉贴近面颊,目的是()
My mother made some posters yesterday.( 改为一般疑问句)
Did your mother ____ ____ posters yesterday?