A radio station charges an initial fee for broadcasting advertisements plus a rate based on the length of the advertisement in seconds. The table below shows the total charges for advertisements of different lengths.

Which equation BEST represents the linear relationship between n, the length in seconds of an advertisement and c, the total charge of the advertisement?

a. c=3n+180
b. c=4n+120
c. c=2n+210
d. c=5n+150

p.s graph is attached