The 5th term of an arithmetic progression is 12 and the 13th term is 28. Which of the following pairs of equations in the first term a and common difference d correctly represents this information?
a) a+5d=12,a+13d=28
b) a+4d=12,a+13d=28
c) a+4d=12,a+12d=28
d) a+4d=28,a+12d=12