Aiden and Zaria Charge People to Park on Their Lawn

Aiden and Zaria charge people to park on their lawn while attending a nearby craft fair. At the current price of $10, six people park on their lawn. If they raise the price to $15, they know that only five people will want to park on their lawn. Whether they have six or five cars parked on their lawn does not affect their costs. From this information it follows that

A) they should leave the price at $10.
B) it does not matter if they charge $10 or $15.
C) they would do better charging $15 than $10.
D) they should raise the price even more.