1 . A small group of paleontologists (古生物学家) recently discovered 10 species of ancient mammals previously unknown to science with the help of an enormous number of helpers at their dig site: ants.

The study of ancient mammals throws new light on the diversity of mammals that existed in North America around 33 million to 35 million years ago, when the climate was changing dramatically. It also pays attention to the harvester ants, with which researchers have long had a love-hate relationship. “The ants are not fantastic when they’re biting you,” said Samantha Hopkins, a professor of Earth Sciences at the University of Oregon. “But I’ve got to appreciate them because they make my job a whole lot easier.”

Most species of harvester ants live in subterranean caves that sit beneath a small hill of dirt. They strengthen the dirt by covering it with bits of rock and other tough materials. The ants have been known to travel over a hundred feet from their caves and to dig six feet deep in pursuit of materials that help secure their caves. The materials include fossils. Harvester ants can carry materials 10 times to 50 times the weight of their body, although they do not weigh very much, so the heaviest fossil they can collect weighs less than the average pill.

Given the size, harvester ant hills are hot spots for what scientists call microvertebrate (微型脊椎动物) fossils, which are animal fossils too small to see without a microscope. For over a century, scientists like Dr. Hopkins have found sediment (沉积物) off the sides of harvester ant hills in search of these fossils, making it easier to find large numbers of fossilized mammal teeth without spending hours in the field sifting through sand and dirt.

1. What is the purpose of the passage?

A．To compare two different species.

B．To provide evidence for discoveries.

C．To promote awareness of mammal protection.

D．To introduce a kind of ant serving as a helper.

2. Which aspect may influence the diversity of mammals in the past?

A．Ant numbers.

B．Climate.

C．Cave materials.

D．Dirt locations.

3. What does the underlined word “subterranean” mean in Paragraph 3?

A．Coastal.

B．Underground.

C．Urban.

D．Mountainous.

4. What can we learn about the harvester ants from the last two paragraphs?

A．Their caves are miles deep.

B．They can carry pills around.

C．Fossils may be found around their hills.

D．Materials with fossils are their food.