Which statement correctly distinguishes a geometric sequence from an arithmetic sequence?

Consider the geometric sequence $5$, $-15$, $45$, ... Identify the common ratio, $r$.

Given the geometric sequence with $a_1=3$ and common ratio $r=2$, compute the $20$^th term, $a_{20}$.

A geometric sequence has $a_2=12$ and $a_5=96$. Determine $a_1$ and $r$, and write the general formula for $a_n$.

A biologist models a population by $P_n=1,000,000\cdot (0.7{)}^n$ where $n$ is hours. Determine after how many whole hours the population will drop below $1,000$ individuals. Determine the minimal integer $n$.