参考答案：C

解析：名词辨析。discord争论、不和：A note of discord crept into their relationship．他们的关系出现了裂痕。discovery发现：a new discovery in medical science医学科学的新发现。rivalry竞争、敌对者：There is great rivalry between the two sisters．这两姐妹间有一场激烈的竞争。cooperation合作：Thank you for your cooperation．感谢你们的合作。