Environmentalists are earnestly trying to determine ________ of the ozone layer over the poles.

Historical records.................... were used in China for centuries before spreading to Europe.

The first mention of the cultivation of tea................ in a Chinese dictionary which dates about the year 350.

...............global climate is warming has been well documented.

...................floats or sinks is determined by its density rather than its weight.

Edmund Halley , who had discovered Halley's comet in 1682 ,--------------before he could again see the comet whose return he had accurately predicted.

Some radioisotopes which .................as radioactive waste are now being used in experimental cancer therapies.

Although seismologists know __________, they are unable to predict with accuracy when or where a tremor might occur.

The abundant grain harvest of North America can be credited to Norman Borlag, a little-known scientist ___________ modern strains of high-yield wheat.

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  ____________________ in the atmosphere is the temperature falling below freezing

• Subduction is the process _________ the ocean floor recycles itself..

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  _______________ is so perfectly with their environment is what makes Frank Lloyd Wright’s architecture so distinctive.

• Bees perform a complex dance ____________ other bees about the location of a good source of nectar.

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  In addition to quitting smoking and controlling blood pressure and diet, people need to think about how ______________ stress to avert potentially fatal heart problems.

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Rarely ________ now used for battle purposes.

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• Only recently _________ popular all over the country.

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• A motionless atom is colder than _________________