Which of the following describes a linear recurrence relation?
an=an−12a_n = a_{n-1}^2an=an−12
an=an−1+3a_n = a_{n-1} + 3an=an−1+3
an=an−1a_n = \sqrt{a_{n-1}}an=an−1
an=2an−1a_n = 2^{a_{n-1}}an=2an−1