Bob ran the 100 meters in 9.91 seconds, and I have not seen _________ this year.

A．the best

B．better

C．the most

D．more

设G=(x，y)|x²+y²≤r²是以原点为圆心，半径为r的圆域，随机变量X和Y的联合分布是在圆G上的均匀分布，证明量X和Y不独立，也不相关．