Pick your answer from:
1). weakness
2). layers of fat
3). a cuticle
4). exercise
5). pain
6). mutations
7). pleasure
8). roots

You are presented with the quiz "What helps species grow stronger?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "mutations are essential for evolution to occur" and "besides, evolution means stronger members of the species survive". So, what's the best answer to the question?
Answer:
6).