The passage is mainly about ( ).

参考解析：

根据首段If your friends are happy, it turns out that you're more likely to be happy too. If your friends are overweight, that too ups the odds you'll pack on pounds. Those effects have been shown in studies......可知如果你的朋友很开心，那么你也更有可能很开心。如果你的朋友超重，那也会增加你发胖的几率。这些效应已在研究中得到证实……尾段In other words, if your prescription is more exercise. The doctors might want to write a prescription for your friends and family, too.可知换句话说，如果你的处方是更多的锻炼，医生可能也会给你的朋友和家人开处方。原文中每段也涉及到了其他人对于自己跑步的影响。选项A：同伴对跑步的影响；选项B：财富对跑步的影响；选项C：锻炼的好处；选项D：锻炼的普及。故本题正确答案选A。