70 miles in 3 hours = 70/3 ≈ 23.33 mph
If 1 mile = 1609 m the 23.33 miles≈ 37543.33 meter per hour.
Divide by 60 to change to meters per minute and again by 60 to change meters per minute to meters per second - Shortcut is to divide by 60 x 60 or 3600
37543.33 ≈ 10.428 meters per second. Answer a. is correct
3600