Ren hana definitely maybe :)
I know that c and d are correct because if you read the paragraph you see that none of the other anwseres relate to the qustion
I usually soak it in hot/warm water for about 30 minutes and then just use a washcloth or something to try and get it out a little at a time and if that doesn't work I soak it in hot soapy water and try it that way. I think the soapy water would do better for you. It usually works for me.
I hope this helps in some way.
Answer: monthly. Three months to be exact
