The correct answer is: There are no more than two products; There are constant unit variable costs and; There are constant total fixed costs.

We assume there are no more than two products so that we can use a two dimensional graph. We assume that there are constant unit variable costs so that contribution per unit does not change as the production level changes. We assume there are constant total fixed costs so that fixed costs can be ignored when calculating the optimal production mix.

There can be any number of limited resources, and the fixed costs per unit will change every time the production quantities change.