下列哪些是 36 和 54 的公因数？

1. List the factors of 36

Factors of $36$: $1, 2, 3, 4, 6, 9, 12, 18, 36$

2. List the factors of 54

Factors of $54$: $1, 2, 3, 6, 9, 18, 27, 54$

3. Identify the common factors and select the correct option

Common factors of $36$ and $54$: $1, 2, 3, 6, 9, 18$
Checking the options:
(1) $2$ and $27$ ($27$ is not a factor of $36$)
(2) $3$ and $12$ ($12$ is not a factor of $54$)
(3) $4$ and $9$ ($4$ is not a factor of $54$)
(4) $6$ and $18$ (Both are common factors)