单选题：阅读下面短文，回答下面（49～52题）．掌握其大意，然后从21～40各题所给的四个选项（A、B、C和D）中，选出最佳选项

阅读下面短文，回答下面（49～52题）．掌握其大意，然后从21～40各题所给的四个选项（A、B、C和D）中，选出最佳选项，并在答题纸上将该选项标号涂黑。 C 　　Plants can’t communicate by moving or making sounds,as most animals do.Instead,plants produce volatile compounds,chernicals that easily chasse trom a liquid to a gas .A flower’s sweet smell,for example,comes from volatile compounas that the plant produces to attract insects such as bugs and bees.
　　Plants can also detect volatile compounds produced by other plants,A tree under attack by hungre insects ,for instance, may give off volatile compounds that let other trees know about the attack,In response,the other trees may send off chemicals to keep the bugs away- or even chemicals that attract the bugs’natural enemies.
　　Now scientists have created a quick way to understand what plants are saying: a chemical sensor(传感器) called an electronise nose,The “e-nose” can tell compounds that crop plants make when they’re attacked.Seientists say the e-nose could help quickly detest whether plants are being eaten by insects,But today the only way to detect such insects is to visually inspect individual plants.this is a challenging task for managers of greenhouses,enclosed gardens that can house thousands of plants.
　　The research team worked with an e-nose that recognizes volatile compounds.Inside the device,13 sensors chemically react with volatile compounds,Based on these interactions ,the e-nose gives off electronie signals that the scentists analyze using computer software.
　　To test the nose,the team presented it with healthy leaves from cueumber,pepper and tomato plants , all common greenhouse crops.Then the scientists colleced samples of air around damaged leaves from each type of crop.These plants had been chaged by insects, or by scientists who made holes in the leaves with a hole punch (打孔器).
　　The e-nose,it turns out,could identify healthy cucumber,pepper and tomato plants based on the volatile compounds they produce,It could also identify tomato leaves that had been damaged.But even more impressive,the device could tell which type of damage – by insects or with a hole punch – had been done to the tomato leaves.
　　With some fine tuning,a device like the e-nose could one day be used in greehouses to quickly spot harmful bugs,the researchers say,A device like this could also be used to identify fruits thet are perfectly ripe and ready to pick and eat,says Natalia Dudareve,a biochemist at Purdue University in West lafayette,Ind,who studies smells of flowers and plants.Hopefully,scientists believe,the device could bring large benefits to greenhouse managers in the near future.
We learn from the text that plants communicate with each other by______．
A．making some sounds
B．waving their leaves
C．producing some chemicals
D．sending out electronic signals