The word ones” in the passage refers to .

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 31 to 35.

Dinosaurs were reptiles that lived during a period of earth’s history called the Mesozoic Era, which is known as the Age of Reptiles. The first dinosaurs appeared more than 200 million years ago. For many millions of years, they dominated the land with their huge size and strength. Then about 65 million years ago, they died out rather suddenly, never to reemerge.
The word dinosaur comes from two Greek words meaning “terrible lizard”. Dinosaurs were not lizards, but their appearance could be truly terrifying. The biggestonesweighed more than ten times as much as a mature elephant and nearly equaled the size of most modern-day whales. The famous kinds of dinosaurs, including the brontosaurus and tyrannosaurus, reached 80 to 90 feet in length. Not all dinosaurs were giants, however, some were actually no larger than a chicken.
Scientists still do not know what caused dinosaurs to disappear. One theory involves a change in the earth’s climate. It is believed that temperature dropped significantly towards the end of the Cretaceous Period. Too large to hibernate and not having fur or feathers for protection, it is possible that the climate became toochillyfor dinosaurs. In contrast, other species having protection, such as the mammals and birds, were able to survive.