选C.句意：一些作物吸收我们呼出的二氧化碳而给我们呼出氧气作为回报.A.in case 万一；假使B.in turn 轮流,依次=by turns,in file；转而,反过来（强调“按顺序轮流过来之后又转向”）C.in return 作为回报；作为…的....in turn反过来（强调“按顺序轮流过来之后又转向”）能就此点给个例句吗，最好来个和in return 作对比的Teachers are more responsive to you, people treat you better; you have more opportunities to make your way through the world; you make more money; you have more access to things,;and that could, in turn, cause you to improve your life. 老师更喜欢回应你，人们待你更好，你有更多的机会闯出自己的一条路，你赚到更多的钱，你能得到更多的东西，从而使你能改善你的生活。If you are alwayshelpingothers, you will be helped in turn when you need help.I just didn't want to take anything If I couldn't offer anything in return. We'd be happy with just one dance. 如果我不能给予回报的话,我不想接受任何东西。如果能跳个舞，我们就很高兴了。We should try our best to our life to the full in return for the kindness of our parents.