①Swifts live mostly in the air. These small birds are among the fastest animals on Earth. In fact, swifts can go two or three days without touching the ground! To get used to life in the sky, swifts use their speed and the saliva in their mouths.

②Swifts can fly faster than 100 miles per hour. That’s because their wings are like boomerangs. This shape of the wings helps them to fly much faster. They can also move one wing faster than the other, which allows them to make sharp turns without slowing down. Making fast, sharp turns helps swifts catching flying bugs to eat.

③The saliva helps swifts catch bugs to feed their young. It is sticky, like glue. Swifts often fly into clouds of bugs such as mosquitoes and open their wide mouths to catch as many insects as they can. The bird collects insects in its mouth graduallyand holds the insects in a sticky ball of saliva. One food ball can contain nearly 1,000 bugs.

④Swifts also use their saliva as glue to build their nests. The birds catch feathers in the air and stick the feathers together to build a strong nest on the side of a cliffor building. Swifts’ short feet are not good for walking or standing on land for long. But their sharp claws allow them to stick to rocks and walls so they can take a brief rest before returning their lives in the sky.

A．	B．
C．	D．

There’s nothing better than a walk in the countryside for lifting our spirits. Connecting with nature can help our mental (精神的) health. For many people, it feels great to take a walk through a forest, but as we admire the trees around us, it’s worth knowing these trees are doing more than just being nice things to look at.

We have already noticed that trees play an important role in taking care of our world—they produce oxygen (氧气) and clean the air we breathe by taking in about a quarter of all human-caused carbon dioxide emissions (二氧化碳排放).

But there’s more to these trees than we might think. Researchers have discovered that they are actually very smart.

It’s thought that trees use a fungal network (真菌网络) to talk and share resources (资源) right under our feet. Under the ground are tree roots, bacteria (细菌) and super fine threads of fungi (菌丝). And research has shown that they are all mixed together. They can help each other by sharing “food”, and they can even tell them some coming dangers. Scientists say it’s like the trees are talking to one another.

Ecologist (生态学家) Suzanne Simard has called this network the Wood Wide Web. She discovered that parent trees use this network to help their children. She also found that trees are smart enough to change their behaviour to live in a changing environment.

So it seems trees really are the stars of our natural world, and with around three trillion (万亿) of them on our planet, it’s time to show them some respect if we want them to grow better.