The Miller-Rabin primality test is a probabilistic algorithm. For a given odd integer $n > 3$, we write $n - 1 = 2^s \cdot d$ where $d$ is odd. If $a^d \equiv 1 \pmod n$ or $a^{2^r \cdot d} \equiv -1 \pmod n$ for some $0 \le r < s$, then $n$ is a 'strong probable prime' to base $a$. For $n = 121$, which base $a=3$ reveals it as composite?