The pizza restaurant down the street wants to advertise on its marquee sign the number of ways you can order a $10 pizza. The restaurant offers 18 toppings and for $10 you can order up to 4 toppings, meaning you can either order 1, 2, 3, or 4 toppings and you can't order any toppings twice. Use this information to answer the following questions:

a. Is this a permutation problem, a combination problem, or a problem using both elements?
b. Determine how many ways you can order a pizza from the restaurant. Explain what strategies you used.
c. If there were no restriction on the number of toppings you could choose, how would that change the answer?