高中英语北师大版（2019）选择性必修第一册Unit 3 Conservation Lesson 2 War on Plastic Packets同步练习

With keepers to feed them, vets to cure them, and no predators (捕食者)to eat them, animals in a zoo usually live much  (long) than animals in the wild. Zoo life is safe, but it can also be boring.

In the wild, survival is  full-time job, consisting of finding food, taking care of babies, and steering clear of all the things that might want to eat them.  (compare) with their wild cousins, says one zoo vet, zoo animals are unemployed.

Scientists  (observe) animals in their natural environments realized that some zoo animals were doing all sorts of  (nature) things those species in the wild don't do. Tigers paced back and forth  (end). Elephants moved their heads up and down. These are signs of boredom and stress. And scientists have learned that being stressed out can actually make animals and people sick.

Giving zoo animals food and shelter  (be) not enough to keep them healthy. They also need something  (do). These days, problems like pacing are less common because zoos try to make life interesting for animals that live there, with habitats, games, and even toys encourage natural skills and behaviors. These things are just as important for the animals' good health  a proper diet or a visit from the vet.

Caleb Arnold loves birds, especially hummingbirds. The 7-year-old boy knows a lot about his favorite animal. "They pollinate (授粉)plants, and they're pretty and small, " he says. But there's one thing Caleb didn't know: hummingbirds see a wider range of colors than we do.

It makes sense that Caleb didn't know this. Few people did for sure, until June 15, 2020, when researchers published a report in Proceedings of the National Academy of Sciences. It confirmed the fact that scientists have long suspected. Birds are experiencing a more colorful world that is above our own. A team of scientists conducted a three-year experiment to find out if hummingbirds are able to tell apart colors that look the same to humans.

To understand the experiment, it's important to know how color vision works. There are three types of tiny color-sensitive cones (锥细胞)in the human eye. Birds, and some other animals, have a fourth. This means they can also see ultraviolet (紫外线的) colors. The ultraviolet cone type allows birds to see combination colors that humans can't, like ultraviolet green and ultraviolet red. Biologists studying birds have supposed that birds can see these, but it's been challenging to test this idea.

That's exactly what the team did. They put two bird feeders in the wild. One contained ordinary water. The other contained sugar water. Both feeders held a specially designed LED light stick—sort of a bird-vision light tube that changed colors. If we are shown the ultraviolet green tube and the green tube, they'd both just look green to us. But the hummingbirds saw the difference. They could even tell apart two shades of ultraviolet red. The scientists observed that the birds were able to use the colors of the lights to find the sweet water.

Caleb is glad that scientists want to better understand birds. "Good," he said, after hearing the news. "They're part of nature."

By the time I was 12, my family had owned many types of pets—a rabbit, countless fish, and even birds. But I always 1 a puppy. So when I returned from sleep-away camp in sixth grade and was 2 by a little dog named Tyler, it was quite possibly the 3day of my life.

A couple of years went by and Tyler was the picture of perfect 4, until we came home from a family vacation to find him acting unlike his 5 happy self. He had no 6 in playing or eating, and he was losing tons of hair and weight.

Filled with 7, we rushed him to the doctor's to run tests. A terrible tumor (肿瘤) was found between his heart and lungs. Every doctor we spoke to 8 he should be put to sleep. We were told that 9 we tried to treat his cancer, he would only live another two to three years.

However, we decided not to give up this little 10 thing. An operation was performed on him and his health was 11 improving, but we still couldn't 12 what the doctors had told us, "Only two to three more years..."So with that in mind, we 13 him like a prince. We loved him like no puppy had14 been loved before—at least in our minds.

Tyler is turning 11 years old this year. The only 15 doctors have offered to us is that he is still alive because of the strong tie he has with my family.