虽然他们能扮演一个激励性和令人满意的角色，但他们越来越必须权衡利益与风险孰轻孰重。
Though they can play a stimulating and satisfying role, they increasingly have to weigh the advantages against the risks.
空气污染越来越严重，所以我们必须采取措施加以控制。
Air pollution is becoming increasingly serious, so we must take measures to control it.
该协会还表示，必须提高机组人员和空中交通管制员的培训标准，因为空中交通越来越拥挤。
The association also said that standards of training must be raised among flight crews and air traffic controllers as the sky has become ever more crowded.
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