An aquarium tank containing 240 gallons of water is being drained at a rate of 40 gallons per hour. The linear function w(x) = -40x + 240 models the amount of water in the tank, where x is the number of hours and w(x) is the total number of gallons in the tank. What is the domain of the function?

a) (-[infinity], [infinity])
b) [0, 6]
c) [0, 240]
d) [0, 40]