Questions 17-20 are based on the following passage.

    The Megatherium is an extinct type of giant ground sloth that lived from two million to perhaps 4,400 years ago. While medium-sized sloths continue to live in arborous habitats in Central and South America, the ground-dwelling Megatherium lived in parts of North and South America. These animals were as big as elephants and had huge claws on their feet. Evidence suggests three theories that may have contributed to the giant sloth’s extinction, which occurred at about the same time as humans arrived on the continent.

    The first theory pertaining to the extinction of the giant ground sloth connects its disappearance with the arrival of humans. The most plausible explanation for this simultaneous actions is that humans hunted the giant ground sloth to its extinction. Early humans hunted animals for a variety of reasons, but food and clothing were among the most important. They could eat the animals that they killed and then fashion the skins into do thing. Humans may also have killed animals as perceived threats, even if they were not valuable as a source of food or clothing. Scientists using radiocarbon to date giant sloth fossils from Cuba and Hispaniola recently found that the last record of a giant ground sloth coincided with the arrival of humans about 4,400 years ago.

    The evidence found in favor of the human hunting hypothesis contrasts with a second theory that climate change was the primary reason for the giant ground sloth’s extinction. Some scientists think that the giant ground sloth wasn't able to adapt to the climate changes that followed the last Ice Age, which ended about 10,000 years aga. At the onset of the Ice Age, the Earth's temperature dropped, and ice sheets and glaciers expanded. These changes would have affected the giant ground sloth’s eating habits, and evidence has been found to prove that the last surviving giant ground sloths had drastically different eating habits than their immediate predecessors.

    A third theory has looked at the possibility of a hyper disease leading to the giant ground sloths extinction. This theory maintains that the giant ground sloth was particularly vulnerable to disease because of its large body size and small population. One suggestion is that the domestic dogs that humans brought with them transmitted pathogens to the giant ground sloth population. Critics of this theory state that it cannot account for several major extinction events that occurred before humans and domestic dogs migrated together. In Australia, for example, domesticated dogs did rot arrive until 35,000 years after humans lived on the continent and 30,000 years after the giant ground sloth was believed to be extinct there.


The overall tone of this passage can best be described as ....