How do we know that is an example of an exponential decay?

A) it’s a smooth curve going up
B) it’s a line going up
C) it’s a smooth curve going down
D) it’s a line going down

Last month, a 770 g box of cereal was sold at a grocery store for $3.10. however, this month, the cereal manufacturer launched the same cereal in a 640 g box, which is being sold at $2.50. what is the percent change in the unit price?