Nick has $9 to spend for breakfast. He decides to head out to his local donut shop for some coffee and donuts. A cup of coffee costs $3 (no refills), while each donut costs $1. Which of the following would represent Nick's breakfast budget constraint? (let x = number of cups of coffee and let y = number of donuts)
Choose one:
A. 3x+1y=9
B. 3x+y=9
C. x+3y=9
D. 3x−y=9