How many ping pong balls would it take to fill a classroom that measures 22 feet by 16 feet by 9 feet? (assume that a ping pong ball has a diameter of 1.5 inches and that the balls are stacked directly on top of each other).

You could look at each dimension seperately:
1.) How often does 1.5 inches fit into 22 feet?
1.5 inches=0.125 feet  (looked up the conversion)
22 feet/0.125 feet= 176
2.) How many balls fit into 16 feet? same thing :)
16 feet/0.125 feet=128
3.) How many balls fit into 9 feet?
9 feet/0.125 feet=72

Then you multiply them all out because there are 72 layers of 176x128 balls, right?
9*176*128=1622016 balls overall
Hope that helped ;)

C. expression because any expression or equation may contain more than one variable

Bis the right answer