What's the meaning of the prefix im- or in-?

The meaning of the prefix im- or in- is not, or opposite of.
For example:
possible - impossible 

B." to guard against " is correct answer

The details about the statues, i believe, are that they're carved from blocks of igneous stone, many of them are partially buried under the ground which to preserve and protect them from the harsh environment on easter island, and many of the facial features have washed away. also, the most well-preserved one is at the british museum in london.these are just guesses from what i can see and what i think i read correctly. i'm sorry i couldn't be more .