Today, ultrasonic(超声的)waves are being put to work in laboratories and factories．If an ultrasound generator is placed in a liquid, the waves move the liquid back and forthhundreds of thousands of times each second．This causes materials to mix quickly or to dissolve(使分解，使溶解)in liquids．Paint manufacturers(制造商)use ultrasound to do a better job of blending colors．The companies that make film for your camera find that mixing chemicals by the use of sound waves will produce a more sensitive film．
The new lightweight(轻量的)type of washing machine uses ultrasonic waves to get clothes clean．Its special ultrasound generator is put into a pail of soapy water containing the soiled clothes．The sound waves drive the soapy water back and forth through the cloth so fast that everything is soon clean．There is also a new kind of dishwasher that works in much the same way．
Ultrasonic waves can shake a liquid so fast that tiny holes form all through it．The liquid is actually torn apart by this action．Almost as soon as these holes are made, they fall together again．The result is a powerful pounding action．In the dairy industry this is used for the double purpose of making homogenized(使均匀，使匀质)milk and sterilizing(使无菌)it at the same time．If you look at some raw milk with a microscope, you find that it is made up of little drops of butter fat floating around in a watery liquid．In order to make milk easier to digest, these fat droplets(微粒)may be broken up by forcing the milk through very small openings．The result is called homogenized milk．When the ultrasonic method is used, the sound waves not only break up the droplets but also kill the germs in the milk by pounding them to pieces．
(1)
Why does a paint manufacturer use ultrasound to do a better job of blending colors?
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A．
Because it is cheap to use ultrasound to blend colors．
B．
Because the waves move the liquid so quickly that it can make materials mix quickly or dissolve in liquids．
C．
Because they can mix chemicals by the use of sound waves．
