michael walks at a rate of 6 feet per second.which is the closest to this rate in mph?

1 mile=5280 feet
1 hour=3600 seconds.

the choices are
a:3.0
b:3.5
c:4.0
d:4.5

If he walks 6 feet/second, that's saying that he walks 6 feet in a second. So he walks 3600x6=21600 feet in 3600 seconds, which is 21600 feet in an hour. But 5280 feet equals one mile, so he's walking 21600/5280 miles per hour. Which of the available choices is closest to that?