You will be given a definition of a task first, then some input of the task.
You are given an elementary science question along with a few answer options. List all the words from the question that are essential and sufficient for choosing the correct answer option. Keep in mind that essential words are necessary to understand the meaning of the given question. However, non-essential words provide little information needed to select the correct answer. You can think about it like this: If the question was in another language, knowing the meaning of which words were necessary and enough for you to understand the question and answer it?. If your answer consists of multiple spans of words from the question, separate those using a comma (", "). Do not generate any word that is not present in the question. Make sure that your response is contained in the given question and the words are in the same order as the question.
 Grammatical correctness is not important.

Question: Fish, like many other water animals, require oxygen in order to survive. The water they swim in contains oxygen and hydrogen. Oxygen reaches parts of the fish's body when ___. (A) the fish drinks the water through its mouth and allows hydrogen into its body (B) water is absorbed through tiny pores in the skin and allows oxygen into the fish's body (C) water passes through the gills, which removes hydrogen from the water and allows it to enter the fish's body (D) water passes through the gills, which removes the oxygen from the water and allows it to enter the fish's body?
Output:
oxygen, hydrogen., Oxygen, reaches, fish's, body