Identify the GCF of the expression 18a3b2−27a2b318a^3b^2 - 27a^2b^318a3b2−27a2b3.
9ab9ab9ab
9a2b29a^2b^29a2b2
3a2b23a^2b^23a2b2
18a2b218a^2b^218a2b2