因式分解:2x2y3-8x4y5
解:原式=2x2y3(1-2xy)(1+2xy)
They decided to leave the waiter a big _____ because the food and service had been excellent.
A.note
B.reward
C.tip
D.gift
叶片上的---------是气体进出植物体的门户,它是由一对---------形的-----------细胞围成的空腔。