The entrance to a mine is 6 feet long by 5 feet wide by 8 feet high. It is being enlarged. If the length increases at 3 feet per hour, the width at 2 feet per hour, and the height at 0.5 feet per hour, how fast is the entrance increasing in ft^3/hour?
a. 135 ft^3/hour
b. 216 ft^3/hour
c. 231 ft^3/hour
d. 1320 ft^3/hour