In 2008, a school's enrollment was 400 students, In 2012, the school's enrollment was 640 students. Which linear model represents the school's enrollment, s, as a function of t, the number of years after 2005? Choose the correct answer below:
a. s(t) =60t +220
b. s(t)=220t +60
c. s(t) =4t +240
d. s(t)=240t +4
e. s(t) =60r
f. s(t)=240