Sentence:  The dawn of the oil age was fairly recent. Although the stuff was used to waterproof boats in the Middle East 6,000 years ago, extracting it in earnest began only in 1859 after an oil strike in Pennsylvania. ... It was used to make kerosene, the <WSD>main</WSD> fuel for artificial lighting after overfishing led to a shortage of whale blubber. Other liquids produced in the refining process, too unstable or smoky for lamplight, were burned or dumped. 
Meaning of the word marked with <WSD> tags:  Chief, most important, or principal in extent, size, or strength; consisting of the largest part.