Câu 25: My radio doesn’t receive the BBC World Service very easily.
A. pick on B. pick out C. pick over D. pick up
Read the passage and choose the best answer from 26 to 35.
It's a sound you will probably never hear, a sickened tree sending out a distress signal. However,
a team of scientists at the U.S. Department of Agriculture's Forest Service has recently heard the
cries, and they think some insects also hear the trees and are drawn to them like vultures attracted to
a dying animal.
Researchers hypothesized that these sound- actually vibrations produced by the surface of plants-
were caused by a severe lack of moisture. They fastened electronic sensors to the bark of drought-
stricken trees and clearly heard distress calls. According to one of the scientists, most parched trees
transmit their plight in the 50- 500-kilohertz range. (The unaided human ear can detect no more
than 20 kilohertz). They experimented on red oak, maple, white pine, aspen and birch and found that
all make slightly different sounds. With practice, scientists could identify the species of tree by its
characteristic sound signature.
The scientists surmise that the vibrations are created when the water columns inside tubes that
run the length of the trees are cracked, a result of too little water following through them.
These fractured columns send out distinctive vibration patterns. Because some insects communicate
at ultrasonic frequencies, they may pick up the trees' vibration and attack the weakened trees.
Researchers are now running tests with potted trees that have been deprived of water to see if the
sound is what attracts the insects. "Water-stressed trees also have a different smell from other trees,
and they experience thermal changes, so insects could be responding to something other than sound",
one scientist said.
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