计算：（2m﹣3n）﹣（m﹣2n）
解：原式=2m﹣3n﹣m+2n=m﹣n．
--- Which of the two English dictionaries will you buy ?
--- I’ll buy ___ of them , so I can give one to my friend , Helen .
A either B neither C all D .both
钠在体内主要分布于（）
A.细胞外液
B.细胞内液
C.骨骼
D.肌肉
E.肝、肾
