OG question type - Help Explain results of study

Hardin argued that grazing land held in common (that is, open to any user) would always be used less carefully than private grazing land. Each rancher would be tempted to overuse common land because the benefits would accrue to the individual, while the costs of reduced land quality that results from overuse would be spread among all users. But a study comparing 217 million acres of common grazing land with 433 million acres of private grazing land showed that the common land was in better condition.


Which of the following, if true and known by the ranchers, would best explain the results of the study?

A) With private grazing land, both the costs and the benefits of overuse fall to the individual user.
B) The cost in reduced land quality that is attributable to any individual user is less easily measured with common land than it is with private land.
C) An individual who overuses common grazing land might be able to achieve higher returns than other users can, with the result that he or she would obtain a competitive advantage.
D) If one user of common land overuses it even slightly, the other users are likely to do so even more, with the consequence that the costs to each user outweigh the benefits.
E) There are more acres of grazing land held provately than there are held in common.


Can someone please help me undestand why: D[/spoiler] is the answer?

Thanks so much!