The answer is B.
The basic plan is c = 20 + 8s ($20 initial cost + $8 each session)
The trial plan is c = 12s ($12 each session)
You need to write a table to determine when the two amounts will equal each other. At month 5 we see that you will end up spending the same amount of money on each plan.
B T
------------
1) 28 12
2) 36 24
3) 44 36
4) 52 48
5) 60 60
