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.

不明白为什么是A？结论说的是a million-dollar contract 的影响，而A说的是small salaries with possible bonuses ，有什么关系么？？

文中的结论：百万美元的合同通过减少经济上的刺激来削弱运动员追求杰出的愿望。（高收入对运动员不利）
A平均来讲，那些低收入的运动员比其他运动员表现的更出色。
D高收入的运动员比低收入的运动员拥有更为成功的职业生涯（高收入对运动员有利,削弱）