Plants can't communicate by moving or making sounds, as most animals do. Instead, plants produce volatile compounds   ---- chemicals that easily change from a liquid to a gas. A flower's sweet smell, for example, comes from such volatile compounds to attract insects such as bugs and bees. Plants can also discover  volatile compounds produced by other plants. A tree under attack by hungry insets, for instance, may give off these chemicals in order to let other trees know about the attack. In response, the other trees may send off their chemicals to keep the bugs away ---- or even chemicals that will attract the bugs' natural enemies. Now scientists have created a quick way to understand what plants are saying: a chemical sensor   called an "electronic nose". The "e-nose" can tell such compounds as plants make. When plants are attacked, scientists say, the e-nose could help quickly decide whether plants are being eaten by insects. But today, the only way to spot such insects is to inspect individual plants by observing them. This is a challenging task for managers of greenhouses, including those that can house thousands of plants. The research team is working with an e-nose that can recognize volatile compounds. Inside the device, 13 sensors chemically react with volatile compounds based on the interactions  , and then the e-nose will give off electronic signals that the scientists can analyze by using computer software. To test the e-nose, the team presented it with healthy leaves from cucumber, pepper and tomato plants, all being common greenhouse crops. Then scientists collected samples of the air around damaged leaves from each type of crop. These plants had been damaged either by insects or by scientists who made holes in the leaves with a hole punch  . The e-nose, it turns out, can 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 can one day be used in greenhouses to quickly spot harmful bugs, the researchers say. A device like this can also be used to identify fruits that are perfectly ripe and ready to pick and eat, says Natalia Dudareva, a biochemist at Purdue University in West Lafayette, India, who studies smells of flowers and plants. Hopefully, scientists believe, the device can bring large benefits to greenhouse managers in the near future. We learn from the text that plants communicate with each other by    _   .
making some sounds waving their leaves producing some chemicals sending out electronic signals
Question: Plants can't communicate by moving or making sounds, as most animals do. Instead, plants produce volatile compounds   ---- chemicals that easily change from a liquid to a gas. A flower's sweet smell, for example, comes from such volatile compounds to attract insects such as bugs and bees. Plants can also discover  volatile compounds produced by other plants. A tree under attack by hungry insets, for instance, may give off these chemicals in order to let other trees know about the attack. In response, the other trees may send off their chemicals to keep the bugs away ---- or even chemicals that will attract the bugs' natural enemies. Now scientists have created a quick way to understand what plants are saying: a chemical sensor   called an "electronic nose". The "e-nose" can tell such compounds as plants make. When plants are attacked, scientists say, the e-nose could help quickly decide whether plants are being eaten by insects. But today, the only way to spot such insects is to inspect individual plants by observing them. This is a challenging task for managers of greenhouses, including those that can house thousands of plants. The research team is working with an e-nose that can recognize volatile compounds. Inside the device, 13 sensors chemically react with volatile compounds based on the interactions  , and then the e-nose will give off electronic signals that the scientists can analyze by using computer software. To test the e-nose, the team presented it with healthy leaves from cucumber, pepper and tomato plants, all being common greenhouse crops. Then scientists collected samples of the air around damaged leaves from each type of crop. These plants had been damaged either by insects or by scientists who made holes in the leaves with a hole punch  . The e-nose, it turns out, can 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 can one day be used in greenhouses to quickly spot harmful bugs, the researchers say. A device like this can also be used to identify fruits that are perfectly ripe and ready to pick and eat, says Natalia Dudareva, a biochemist at Purdue University in West Lafayette, India, who studies smells of flowers and plants. Hopefully, scientists believe, the device can 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
Answer: