填空题 Even though the majority of Americans would find it hard to imagine
what life could be like without a car, some have begun to realize that the
automobile is a mixed blessing. Traffic accidents are increasing steady, 67. ______
and large cities are troubled by too many automobiles. First of all, 68. ______
perhaps, is the air pollution caused by the internal-combustion (内燃机)
engine. Every car engine burns hundred of gallons of fuel each year and 69. ______
pumps gases into the air. These gases are one origin of the smog that 70. ______
hangs over large cities. Some of these gases are poisonous and danger to 71. ______
health, especially for someone with a weak heart or a respiratory disease
One answer to the problem of air pollution is to build a car that does
not pollute. That's the several major automobile manufacturers are trying 72. ______
to do. But building a clean car is easier saying than done. So far, progress 73. ______
has been slow. Another solution is to eliminate car fumes altogether for 74. ______
getting rid of the internal-combustion engine. Inventors are now working
on turbine-powered cars, as long as on cars powered by steam and 75. ______
electricity. But most of us won't be driving cars run on batteries or
boiling water for a while yet. Many auto makers believe that it will spend 76. ______
years to develop practical models that are powered by electricity or steam.