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.
像天然氣這樣的可以在內燃機里燃燒的替代燃料，能夠以相對較低的價格引入，但是它們也只能略微地減輕污染和減少溫室氣體的排放。

六級詞匯講解:
本句的主干是Altemative fuels...could be introduced...，but they would lead to only marginal reductions...。such as natural gas為插入語，是對fuels的解釋說明；bumed in intemal-combustion engines為過去分詞短語作定語，修飾fuels。altemative意為“可選擇的；替代的”。如：
What altemative sources ofgoods and services are there?
有哪些可替代的商品和服務來源呢？
intemal-combustion engine指“內燃機”。relatively意為“相對地”。marginal意為“邊緣的；臨界的”。如：
For example, "marginal" areas must achieve attainment within three years.
比如說，“邊緣”地區必須在三年之內達標。

英語六級考點歸納
與lead相關的常用表達還有：
lead up to sth.意為“由一系列的步驟而導致”。如：
These circumstances willlead up to war.
這些事件會導致戰爭。
lead sb. by the nose意為“牽著某人的鼻子走；完全控制某人”。如：
The manager always leads the clerk by the nose.
經理總是牽著那個職員的鼻子走。
lead a dog's life意為“（某人）過著痛苦不堪的生活”。如：
After the cruel war, he lead a dog's life.
戰后，他過著痛苦不堪的生活。