|A) fish are cold-blooded animals
|B) ice is a bad conductor of heat
|C) there will always be water just beneath the ice level
|D) they can adapt themselves to live in ice
Water has the maximum density at . So at this temperature, it sinks down to bottom and stays here. If it cools further, it will become less dense and move up. So while the upper layers can cool enough to form ice, the bottom layers remain as water at .
Therefore, (C) is correct.