Because there are more than 2 lengths for the sides and because the small triangle faces are right angles we will use this formula:
V=(1/2)bhl
which is equivalent to V=Al (A=(1/2)bh and A is Area)
b=12in
h=35in
l=40in
A=(1/2)•12in•35in
A=(1/2)•420in
A=210in^2
V=Al
V=(210in^2)•40in
V=8400in^3
Making your answer D. 8400 inches cubed