Question

Which formula can be used to describe the sequence below?
$27,9,3,1, \frac{1}{3} \text {, }$
$a_n=-3 a_{n-1} ; a_1=27$
$a_n=\frac{1}{3} a_{n-1} ; a_1=27$
$a_n=a_{n-1}-3 ; a_1=27$
$a_n=3 a_{n-1}-\frac{1}{3} ; a_1=27$