The University of Nottingham study examined whether the possibility of punishing free-riders leads to better group results.

VOA: special.2009.03.03

What do you think about free riders?

你对免费搭车者有什么看法？

怎么协调意见冲突 - SpeakingMax英语口语达人

But there are ways to prevent free riders.

但搭便车问题，也有办法避免。

耶鲁公开课 - 公正课程节选

After watching for some time, I decided the three riders must be hostile Indians.

VOA: special.2010.07.10

In one event, riders try to stay on a wild animal for eight seconds.

VOA: special.2009.02.16

He organized a force of horse soldiers known as the "Rough Riders."

VOA: special.2010.08.05

I soon found the trail left by the three riders following Powell.

VOA: special.2010.07.10

It used a method of attaching "riders" to legislative proposals.

VOA: special.2010.03.11

The group became known as Roosevelt's "Rough Riders."

VOA: special.2010.07.15

The parade includes marching bands and horseback riders.

VOA: special.2009.12.28

Letters were carried by riders on fast horses.

VOA: special.2009.05.28

Riders use an electronic key to unlock a bike from one of more than 300 stations.

VOA: standard.2010.07.30

Out back,you'll often see small flocks of sheep or goats being herded by riders on horseback.

VOA: standard.2010.03.22

The riders and horses undergo rigorous training, including how to deal with the public in a variety of situations such as huge crowds of Olympic fans.

VOA: standard.2010.02.25

It's a beautiful, wild trail for hikers,bikers, horseback riders - and canal lovers.

VOA: standard.2010.04.29

We're going to have 16 riders, all original airmen,"

VOA: standard.2009.12.30