Which of the following best defines a mixture problem in algebra?

• A. A problem limited to counting identical discrete items, where the total number is known and each item is the same, so there is no need to model differing unit values.
• B. A problem that always requires solving a system of three or more nonlinear equations describing chemical reactions and conservation of mass, where the variables cannot be separated algebraically. This type of problem is used only in advanced chemistry applications, where units are omitted and proportions are assumed to be equal.
• C. Any arithmetic exercise involving only percentages, without variables or equations, and often solved by mental math when exact calculation is not required.
• D. A problem in which two or more distinct quantities (or parts) with measurable properties are combined, and you must model their contributions so that the sum of those contributions equals a given total.

Answer: (D)