${6\choose 2} = \frac{6!}{2!(6-2)!} =15$ 
At an ice cream shop, a sundae is made by selecting two flavors of ice cream and topping them with fudge, whipped cream, nuts, and a cherry. The available flavors of ice cream are chocolate, vanilla, strawberry, rocky road, chocolate chip cookie dough, and mint chip. How many difference sundae combinations are possible? 
Options:
a) $$2$$ 
b) $$15$$ 
c) $$24$$ 
d) $$120$$ 
e) $$720$$ 