A baseball diamond is a square whose sides are 90 ft long. If the batter runs at the first base at the rate of 20 fps, how fast is his distance from second base changing when he has run 50 ft?

A baseball diamond is a square whose sides are 90 ft long. If the batter runs at the first base at the rate of 20 fps, how fast is his distance from second base changing when he has run 50 ft?

a. 80/971/2

b. -10/901/2                               

c. -20/971/2                                       

d. -90/971/