| 207-What the passage stresses about Sherpas is that they………… .
A)are members of a religious sect trying to ban mountain-climbing in Nepal
B)climbed high mountains until the government forbade them in the 1960s
***C)are essential to climbers who want to conquer the mountains in Nepal
D)don't approve of Europeans and Americans climb their mountain
E)want to be the first people to climb Machhapuchhare
Plants can summon an insect rescue team when they are attacked by pests, just as if they were calling for a microscopic ambulance. In fact, researchers say the signal is specific enough to tell the helpful insects exactly what to expect when they arrive on the scene. For example, two kinds of caterpillars attack numerous crops and cost US farmers about $6 billion annually. The plants summon a black, parasitic wasp that it is the natural enemy of the caterpillars. Scientists have known for years that plants could send out distress calls to wasps and other insect bodyguards, but they are just beginning to understand how sophisticated the messages can be. They hope to find out more about the signals and eventually use them to develop chemical-free pest control systems.
Share with your friends: |
