题目内容：

    A group of scientists trained dogs to detect(发现)a crop disease called citrus greening. The disease has affected orange, lemon and grapefruit trees in the American states of Florida, California and Texas. The dogs can detect the disease weeks to years before it appears on tree leaves and roots, the researchers report. Using dogs is also faster, less costly and more exact than having people collect hundreds of leaves for lab analysis.

Timothy Gottwald is a researcher in the study. He said, “This technology is thousands of years old — the dog's nose. We’ve just trained dogs to hunt new prey：the bacteria that cause a very damaging crop disease.”

Citrus greening is caused by a kind of bacteria that are spread by a tiny insect that feeds on the leaves and stems of citrus trees. Once a tree is infected(感染)，there is' no cure. The disease has also hurt citrus crops in Central and South America and Asia. In one experiment involving grapefruit trees in Texas, trained dogs were correct 95 percent of the time in telling the difference between newly infected trees and healthy ones. “The earlier you detect the disease, the better chance you have at stopping healthy trees being infected by removing infected ones,” Gottwald said.

Professor Matteo Garbelotto says the new research shows that dogs can detect an infection well before present methods. He has been involved in similar research but had no part in the new study.

Laura Sims is a plant scientist. She praised the steps taken to find out if the dogs were detecting the bacteria itself or a plant's reaction to an infection. To do that, the researchers infected different kinds of unrelated plants with the bacteria in a laboratory. The dogs were still able to pick out the infected plants.

Gottwald said, "You've seen dogs working in airports, detecting drugs and explosives. Maybe soon you will see them working on more farms."

1.What does the text say about citrus greening?

A.It mainly damages tree branches.

B.It' s impossible to detect at an early stage.

C.Dogs can better detect it than humans.

D.Humans have to analyze tree roots to detect it.

2.How does a dog detect citrus greening?

A.By tasting. B.By touching.

C.By observing. D.By Sniffing.

3.Early detection of citrus greening enables farmers to ______.

A.help infected trees to recover B.keep it from spreading further

C.improve the quality of fruit. D.better protect infected trees

4.Which of the following statements reflects Gottwald's opinion?

A.Dogs will take the place of humans to conduct risky jobs.

B.Trained dogs make no mistakes in citrus greening detection.

C.Dogs use plants' reaction to an infection to help with their detection.

D.Using dogs for citrus greening detection will be more widespread.