What is the volume a of the figure below? ​ ​ A 121212 ft2\mathrm{ft}^{2}ft 2 B 36 ft236\ \mathrm{ft}^{2}36 ft 2 C 54 ft254\ \mathrm{ft}^{2}54 ft 2 D 90 ft290\ \mathrm{ft}^{2}90 ft 2