第255句：

Alternative fuels such as natural gas, burned in internal- combustion engines, could be introduced at relatively low cost, but they would lead to only marginal reductions in pollution and greenhouse emissions.
像天然气这样的可以在内燃机里燃烧的替代燃料，能够以相对较低的价格引入，但是它们也只能略微地减轻污染和减少温室气体的排放。

六级词汇讲解:
本句的主干是Altemative fuels...could be introduced...，but they would lead to only marginal reductions...。such as natural gas为插入语，是对fuels的解释说明；bumed in intemal-combustion engines为过去分词短语作定语，修饰fuels。altemative意为“可选择的；替代的”。如：
What altemative sources ofgoods and services are there?
有哪些可替代的商品和服务来源呢？
intemal-combustion engine指“内燃机”。relatively意为“相对地”。marginal意为“边缘的；临界的”。如：
For example, "marginal" areas must achieve attainment within three years.
比如说，“边缘”地区必须在三年之内达标。

英语六级考点归纳
与lead相关的常用表达还有：
lead up to sth.意为“由一系列的步骤而导致”。如：
These circumstances willlead up to war.
这些事件会导致战争。
lead sb. by the nose意为“牵着某人的鼻子走；完全控制某人”。如：
The manager always leads the clerk by the nose.
经理总是牵着那个职员的鼻子走。
lead a dog's life意为“（某人）过着痛苦不堪的生活”。如：
After the cruel war, he lead a dog's life.
战后，他过着痛苦不堪的生活。