Select from the following.
(1). They have cold blood;
(2). protect them;
(3). They eat a lot;
(4). hard outer covering;
(5). by indirect light;
(6). They go swimming;
(7). They have thick fur;
(8). When it's cold;
Given those answer options, answer the question: You are presented with the quiz "How do polar bears stay warm in the winter?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "some animals' fur grows thicker for staying warm in the winter" and "polar bears grow fur to survive in the cold". So, what's the best answer to the question?
A:
(7).