Provide your best answer for the following math problem. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The problem is: what is the greatest positive integer q such that 3 ^ q is a factor of 9 ^ 10 ?
q = 20
3, 4, 5, or 6.