This is a test of commonsense with single/multi-choices. Complete the next sentence:

How to clean your retainer
Rinse off the retainer with warm or cold (not hot) water.
Place the retainer in a shallow cup, big enough to fully hold it.
Pour vinegar into the cup until the retainer is fully submerged.

pick from the following. (i) Squirt some removable metal adhesive, such as acrylic or polyurethane, onto the retainer. Secure the retainer with some foam sealants, if desired. (ii) Let it sit for 2 to 5 minutes. Longer is not necessary, as the vinegar could begin to degrade the plastic. (iii) Alternatively, you can warm the retainer instead. Strain out excess water using a strainer or a fine strainer. (iv) Tea tree oil and camphor can also be used. Place the retainer in the bag.
The answer is:
(ii)