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

How to tile over tile
Check for loose tiles.
Lightly tap on each old tile with a wood mallet. If the sound is solid, the tile is fine.

Choose from:
-- If the sound seems hollow, the tile is loose and needs to be fixed. Chip away the old caulk or grout around the tile and use a crowbar to lift up the old tile.
-- If it's not, use your fingers to dig deeper. You'll need to try a second tile after measuring it and moving it over any loose tiles.
-- If there are loose tiles, they'll need to be replaced. Splitting a tile by breaking it over tile will give the whole room a shiny appearance and uniformity.
-- Tile dissolves as new tile comes in. If you're replacing tiles most of the time, you can still fix the gap by stripping newly repaired tiles out.
The answer is:
If the sound seems hollow, the tile is loose and needs to be fixed. Chip away the old caulk or grout around the tile and use a crowbar to lift up the old tile.