When we divide the number of feet (~700) by the number of seconds (~20), we get a number that is more than 30 (ft/s). Thus, only choices B and D are viable.
When we invert that rate, we get 1/(30 ft/s) = (1/30) s/ft, leaving the only viable choice as ...
B. 32.6 ft/s and approximately 0.03 s/ft
______
Units (feet, seconds) multiply and divide like any other variable. It is often useful to keep the units with the numbers of a problem so you can tell if you've done the math properly. It is more likely to be correct if the units are correct.
