Denise places seven grams of yeast in a Petri dish, and the mass of yeast in the dish increases 3 grams per day for the next thirty days. Which of the following functions gives the mass M of the yeast as a function of the number of days d?
1) M = 7 + 3d
2) M = 7 - 3d
3) M = 7 * 3d
4) M = 7 / 3d