在空间直角坐标系中,点A(-3,1,4)关于原点的对称点的坐标为 [ ]
A.(-3,-1,4)
B.(-3,1,-4)
C.(3,1,4)
D.(3,-1,-4)
答案:D
I saw Mrs. Smith __________ some flowers in her garden yesterday afternoon.[ ]
A. grows
B. grew
C. growing
D. to grow
吸收塔中段回流是下一层塔盘上抽出液体,经冷却后打入上一层塔盘。