Answer's
16.) Before - You must prepare these item's BEFORE a volcanice eruption start's because these are the material's you will need in case you have to evacuate.
17.) [I don't know the answer to this one sorry!]
18.) After - After the volcanic eruption there are dust's among house's that you must clean because it might be hazardous.
19.) Before - You also need to prepare water because again you might have to evacuate your house/place, then you'll have water to drink safely.
20.) During - It's also important that while waiting in a safe place you have to listen to the radio to be informed about what's happening in your area to be secure when you come back.
