It's ________ good feeling for people to admire the Shanghai World Expo that gives them ________

pleasure.  [ ]

A. /; a     

B. a; / 

C. the; a  

D. a; the

已知地球半径为R，地球表面大气压平均值是p₀，请估计地球周围大气层重力为______（提示：地球体积V=