13. In general, a professional athlete is offered a million-dollar contract only if he or she has just completed an unusually successful season. However, a study shows that an athlete signing such a contract usually suffers a decline in performance the following season. This study supports the theory that a million-dollar contract tends to weaken an athlete’s desire to excel by diminishing his or her economic incentive.

Which of the following, if true, would most strengthen the conclusion drawn above? (A) On the average, athletes whose contracts call for relatively small salaries with possible bonuses for outstanding achievement perform better than other athletes.

(B) Athletes are generally offered million-dollar contracts mainly because of the increased ticket sales and other revenues they generate.

(C) Many professional athletes have careers marked by year-to-year fluctuations in their overall levels of performance.

(D) On the average, higher-salaried athletes tend to have longer and more successful professional careers than do lower-salaried athletes.（A）

(E) Six of the ten leading batters in the National League this season signed million-dollar contracts during the off-season.

这题我是作对了的，可是可能是因为做题太多了的缘故，连自己的搞不清逻辑关系了，不知道什么推什么了，只是凭直觉知道就是A对，请那位NN把原题目的逻辑关系和A是如何加强的方式列一下。

thanks!

题干是说只有运动员的一个赛季成功后，才会有下赛季的很好的 a million-dollar contract。但是研究显示签了这种contract的人在下个赛季会退步，从而证明这种合同会削弱经济上的激励性，从而影响运动员的成绩。如果一些contract的经济效益是以在当期好成绩基础上的bonus体现的，那么签了这些合同的运动员成绩就会好于其他的，是conclusion的一个加强。

谢谢鹿丸大虾。自己读，怎么也搞不清楚逻辑关系，但别人一解释，就明白了。我该怎么提高逻辑水平啊？给点建议，谢谢了！