Molly runs a shop with two products. product a has a contribution margin of $300 and product b has a contribution margin of $100. in january she sold a total of 2,000 units with product a making up 20% of total sales and product b making up 80% of total sales. in february she sold 1,800 units, with product a making up 30% of sales and product b making up 70% of sales. in march she sold 1,500 units, with product a making up 50% and product b representing the other 50%. during which month was molly’s net income the highest?
a : in january, because product b made up the highest percentage of sales out of all three months.
b : in january, because she sold the highest number of units out of all three months.
c : in march, because she sold the highest percentage of product a.
d : in february and march, because the net income for these months is equal.