Question 13If $a_1,$ $a_2,$ $a_3,$ $\ldots$ is a sequence of positive numbers such that $a_{n+2}=a_na_{n+1}$ for all positive integers $n,$ then the sequence $a_1,$ $a_2,$ $a_3,$ $\ldots$ is a geometric progressionfor all positive values of $a_1$ and $a_2$if and only if $a_1=a_2$if and only if $a_1=1$if and only if $a_2=1$if and only if $a_1=a_2=1$