马的脚趾数由四趾逐渐演化成单趾，造成这种进化的原因是（　　）

A．马常用中趾而不用其他趾

B．人类育种

C．自然选择

D．古代马与别种动物交配产生新种

A certain ear increased its average speed by 5 miles per hour in each successive 5-minute interval after the first interval. If in the first 5-minute interval its average speed was 20 miles per hour, how many miles did the car travel in the third 5-minute interval ？（）

A．1.0

B．1.5

C．2.0

D．2.5

E．3.0