Which inequality models this problem? Nico finds two online stores both sell the same kind of party hats that he wants. Store A sells the hats for $7 each and does not charge for shipping. Store B sells the hats for $4 each and charges a flat rate of $9 for shipping.

What is the minimum number of hats Nico will need to buy for the cost to be less at store B than store A?

a. 7x<4x+9
b. 7x>4x+9
c. 7x<4x−9
d. 7x>4x−9