Don't forget to _____"Thank you" when someone has helped you. [ ]

A. speak

B. tell

C. talk

D. say

如图所示，小明把一纸条靠近嘴边，在纸条的上方沿水平方向吹气时，纸条会向                （选填“上”或“下”）偏移，这个现象说明，气体流动时，流速                 的地方压强小。