We all scream for water when thirsty.（15[A]）But do you know in very hot，dry weather plants also make faint sounds—as if they’re crying out for help？
You see，in a plant’s stem there’re hundreds of ’water pipes’that bring water and minerals from the soil all the way up to the leaves.As the ground turns dry，it becomes harder and harder for the plants to do this.
In severe droughts，plants have to fight to pull out any water available.Scientist Robert Winter has found out that when it is really bad their water pipes snap from the tension like rubber bands.When that happens，the whole plant vibrates a little.The snapping pipes make noises 10，000times quieter than a whisper.
Robert knows that healthy，well-watered plants are quiet.（16[D]）He also knows that many insects prefer attacking dry plants rather than healthy plants.How do the insects know which are healthy plants and which are not？Robert thinks that the insects may listen for the plants that cry.And then they may buzz in to kill.
To test his theory，Robert is using a device that can imitate plant cries.He attaches it to a quiet，healthy plant so the plant sounds thirsty.Then he watches insects lo see if they attack more often than usual.
If he is right，scientists could use the insects’ability against them.（17[D]）They could build traps that imitate crying plants.So when the insects buzz in to eat，they won’t buzz out.

What do plants do when they are thirsty?（）

A．They give out faint cries.
B．They make noises to drive away insects.
C．They extend their water pipes.
D．They become elastic like rubber bands.