Which of these is a linear recurrence of order 2?
an=3an−1a_n = 3a_{n-1}an=3an−1
an=2an−1+3an−2a_n = 2a_{n-1} + 3a_{n-2}an=2an−1+3an−2
an=an−1+an−2+an−3a_n = a_{n-1} + a_{n-2} + a_{n-3}an=an−1+an−2+an−3
an=5a_n = 5an=5