A business has a budget of $720 to spend on their marketing campaign. Each month, x, the business spends $40 on different advertisements. The amount of money remaining in the budget for marketing can be modeled by the function M(x) = −40x + 720. Based on the graph of the linear function M(x) and the context of the problem, what is the domain?

[0, 18)
[0, 720]
[0, ∞)
ℝ