For those of you concerned about practical usage, do not be. These are learning exercises in noun and verb agreement, not in how a native Brazilian would speak or write. Too much too soon would be confusing, intimidating and in many cases discouraging. Go with the flow and you soon will be rewarded.
Milk Contamination • Boiled milk may be one of the most ancient cooked foods. Milk and other consumption which doesn't avoid, eliminate, neutralize spoiling and contamination has been, and still can be, the source of dire health.
Many contemporary travelers quite conscientiously restrict their consumption to high heat prepared foods, for a reason.
They are both correct. As a native speaker of US English, I would say "The milk is boiling" for the sentence as given here. I would use "the milk boils" in a sentence such as: "When you turn up the heat, the milk boils," because in this case, "boils" really tells what will happen in the future. In a sentence like that, "the milk is boiling" would not sound right because it is present tense. So it depends on the context.