Which sentence from the passage most strongly supports the author's overall purpose in writing? Livestock, such as sheep, cattle, and goats, are vulnerable to many threats, and predation of valuable livestock costs millions of dollars per year. A guard animal lives with a herd or flock to protect the livestock and deter predators. Shepherds, farmers, and ranchers consider the ideal guard animal to be one that is effective, fearless, cost-effective, and easy to maintain. Surprisingly, guard llamas have proven to fit the bill in most circumstances and can be found guarding animals on farms and ranches across the United States and North America.
Llamas are members of the South American Camelidae family and are used as beasts of burden in many areas of the world. They are well-socialized to humans and bond quickly with livestock. Additionally, llamas react aggressively to members of the Canidae family—including dogs, coyotes, jackals, foxes, and wolves. These attributes make llamas ideal guard animals for a variety of livestock, although they are most widely used to guard sheep. Once llamas become attached to their livestock, they actively protect the animals by chasing, kicking, or spitting at predators and moving the flock to a safe location.
In a study conducted by Iowa State University in 1990, llamas were rated as effective guard animals by 80 percent of sheep owners. Many ecologists and wildlife biologists are also pleased with this practice because it provides a non-lethal way to handle predators. Llamas are also a sound investment for shepherds, farmers, and ranchers because they are long-lived (often living up to 20 years), acclimated to a wide variety of climates, and able to graze on the same land as sheep, goats, and cattle. Although they are not 100% successful against predators, guard llamas can be an effective, economic, and ecologically friendly resource to help livestock owners protect their livelihood.

A. Llamas are members of the South American Camelidae family and are used as beasts of burden in many areas of the world.
B. In a study conducted by Iowa State University in 1990, llamas were rated as effective guard animals by 80 percent of sheep owners.
C. A guard animal lives with a herd or flock to protect the livestock and deter predators.
D. Shepherds, farmers, and ranchers consider the ideal guard animal to be one that is effective, fearless, cost-effective, and easy to maintain.