[简答题]截止阀主要由阀杆、阀芯、阀体、阀片零件组成。（ ）

{{B}} A Delicate Balance{{/B}}
In 1965 the American statesman Adlai E Stevenson said, "We all travel together, passengers on a little spaceship, dependent on its vulnerable supplies of air and soil. We manage to survive by the care, work, and love we give our fragile craft." Our planet is indeed fragile. Every living thing on this planet is part of a complicated web of life, for no organism lives entirely on its own. Every organism is affected by all that surrounds it whether living or nonliving. And in turn each organism has some effect on its surroundings.
Even the most elementary understanding of ecology requires knowledge of this cause/effect relationship all organisms have on each other. Every thing we do to our environment will in one way or another affect the quality of life we experience on this tiny spaceship. If we want the quality of life to be high, we must be more aware that nature is a finely balanced mechan