A:She was learning Chinese.
B:She was preparing for a report.
C:She was helping Betty.
I can sing the song, but he ____.
若x,y满足约束条件 {x−y+1≥0x+y−3≥0x−3≤0 ,则z=x﹣2y的最小值为____.
人类的胚胎在母体内发育的时间大约是