Scientists in Israel have discovered a new way to test for water pollution by "listening" to what the plants growing in water have to say．By shining a laser beam(激光束) on the tiny pieces of algae(海藻) floating in the water, the researchers said they hear sound waves that tell them the type and amount of pollution in the water．
"It is a red light, telling us that something is beginning to go wrong with the quality of water," said Zvy Dubinsky, an aquatic biologist at Israel's Bar Ilan University．"Algae is the first thing to be affected by a change in water quality．"
As water sources deteriorate worldwide, the testing of algae could be used to monitor water quality faster, more cheaply and more accurately than techniques now in use, Dubinsky said．The secret is to measure the rate of photosynthesis(光合作用) in the algae, meaning the plant's ability to transform light into energy．During photosynthesis, plants also release oxygen into the air．
Dubinsky's technique is easy to perform because of the over-abundance of algae in the planet's water．Most of the oxygen in the atmosphere comes from algae．A prototype tester shoots a laser beam at water samples to stimulate photosynthesis in the algae．But not all of the laser's heat is used．
Depending on the condition of the algae and the rate of photosynthesis, some of the heat is shot back into the water, creating sound waves, Dubinsky said．With a special underwater microphone, researchers are able to analyze the strength of the sound waves and determine the health of the algae and the condition of the surrounding water．
"Algae suffering from lead poisoning, like waste discharged from battery and paint manufacturing plants, will produce a different sound than those suffering from lack of iron or exposure to other toxins," said researcher Yulia Pinchasov．
72．To study water quality, scientists choose algae mainly because       ．
A．algae is most easily affected by pollution
B．most oxygen is produced by algae
C．the amount of algae is decreasing
D．algae is a kind of plant floating in the water
73．When Dubinsky studied water quality, he mainly focused on       ．
A．the amount of laser used by algae
B．the rate of photosynthesis in the algae
C．the condition of the algae
D．the heat of laser used by algae
74．After the study, scientists proved that       ．
A．the increase in population is caused by lack of water
B．global warming causes water resources to reduce worldwide
C．algae can show the type and amount of pollution in the water
D．algae is dying out in the ocean around the world
75．What would be the most suitable title for the passage?
A．Water pollution is reduced with the growth of algae
B．Pollution makes it hard for plants to make photosynthesis
C．Water pollution affects the growth of marine plants
D．Scientists listen to plants to find pollution

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