Denote by $\mathbb{Z} / 18 \mathbb{Z}$ the ring whose elements are the integers modulo $18$.

(a) Determine all units of $\mathbb{Z} / 18 \mathbb{Z}$. Enter them as a comma-separated list.

(b) Determine all nonzero* zero-divisors of $\mathbb{Z} / 18 \mathbb{Z}$. Enter them as a comma-separated list.

*(Some authors define zero to be a zero-divisor, others disagree.)