Answer:
Using similar triangles, we can set up a proportion:
(tree height)/(tree shadow) = (building height)/(building shadow)
Plugging in the given values, we get:
8/4 = 24/x
Solving for x, we get:
x = (24 x 4)/8 = 12 feet
Therefore, the length of the building's shadow is B. 12 feet.