Under her cell phone plan, Mariana pays a flat cost of $47.50 per month and $3 per gigabyte. She wants to keep her bill under $80 per month. Which inequality can be used to determine x, the maximum number of gigabytes Mariana can use while staying within her budget?
a. 3(47.5+x)<80
b. 3(47.5+x)>80
c. 80>47.5+3x
d. 80<47.5+3x