2x+5y+4z=0 ①{ 3x+y-7z=0 ②由①得z=-1/2x-5/4y ③将③代入 x+y-z 得x+y+1/2x+5/4y化简得 2x+3y ④又将③代入②化简得 x+3/2y=0 ⑤而⑤×2得④即 2x+3y=x+3/2y=0所以选A