Let sav denote an individual's savings rate, inc their income, and let fem be a dummy variable that equals one for females and zero otherwise. Suppose we want to study how the savings rate depends on income. We want our model to allow for the marginal effect of income on savings to: (i) Be different for men and women, and (ii) Depend on the level of income. Which regression has these properties? O a. sav = B1 + Byfem + Byinc2 + e. Ob. sav = 1 + Byfem + Bzinc + e. O c. sav = Bi + Bzinc + 83(inc x fem) + e. O d. sav = 61 + fzinc" + $3(inc x fem) + e. O e. None of the other options is correct