The first thing we must do for this case is find the amount of BTU produced by the 20 people.
We have then:

Then, we must find the volume of The ice sand, which we can model as a rectangular prism:

Where,
l: length
w: width
h: height
Substituting values we have:

Then, we look for the BTU rate per cubic foot.
We have then:
Answer:
B. .525 BTUs per cubic ft