A football team has a 5 gallon water jug.  Before each practice, Trevor is in charge of filling it up.  At any given time, how many gallons, g, will Trevor have to pour into the water jug given the fraction of the water jug, w, already filled?

a. g= 5/w

b. g=5-5w

c. g=5w

d. g=5-5/w