What would happen if plants are treated badly？Would plants shout when they are stressed（有压力的）？These questions have been explained in a recent paper in Cell.
    "Plants interact （互动） with insects and other animals all the time，and many of them use sounds to talk with each other，so it would be very difficult for plants not to use sounds at all，"Lilach Hadany，one of the writers of the paper，told Science Focus.
    With this in mind，Lilach Hadany and his team started the experiment （实验） by recording plants' sounds.They put tomato plants in boxes to cut off the noise in the environment.Several microphones were then put near them，which can record sounds between 20 and 250 kilohertz，while the sound a human adult's ears can hear is about 16 kilohertz （千赫）.
    There were three groups of plants：One group wasn't watered for five days，another group had the stems（茎） cut off，and the last group was untouched.The microphones caught sounds between 40 and 80 kilohertz from the first two groups，while that's only 25 kilohertz from the third group.It's clear that the sounds grow louder and louder when the plants are stressed.A stressed plant produced the sound around 30 to 50 times per hour，while the untouched group only produced around once per hour，according to the study.
    After the experiment，the team collected the data on stressed plants and then made a machine that could tell how plants "feel".It could tell the differences between unstressed plants，thirsty plants and cut plants.Scientists say that it can be useful for farmers to take better care of their crops.

（1）How does the writer begin this passage？

B

B

A．By explaining reasons.
B．By asking questions.
C．By showing examples.
D．By introducing backgrounds.
（2）The underlined word "them" in Paragraph 3 refers to

D

D

.
A．scientists
B．boxes
C．microphones
D．plants
（3）From the passage，we can know that

C

C

.
A．only tomato plants can make sounds
B．we can hear sounds plants make
C．untouched plants produce fewer sounds than stressed ones
D．the more stressed plants are，the lower sounds plants make
（4）The purpose of the experiment is to

B

B

.
A．help plants interact with animals
B．take care of plants
C．make a machine
D．record plants' sounds