1. Mary buys commodity $x$ and $y$ that maximize her utility $U(x, y) = ax + y$ with $a > 0$. Let her income be $I$, the price of $x$ be $px$, and the price of $y$ be $py$. Answer the following questions in terms of $a, px, py$ and $I$. (12%)

(a) \((2\%)\) Explain why \(x\) is an economic good for Mary.