What is archived through the separation of crude oil - a. crude oil is cracked into different hydrocarbons. - b. crude oil is blended to make different products. - c. crude oil is separated into other fossil fuels. - d. crude oil is distilled into fractions of different molecular weights.

I think it's A but here's an explanation anyway: as you heat the crude oil, the different hydrocarbons, having different boiling points, boil and evaporate up the column. Those with lower boiling points, shorter in  chain length, will condense nearer the top of the fractional distillation column. Those that are more viscous, larger in chain size, will have high boiling points, so will evaporate last and condense lower down in the column. Therefore, you achieve all the different oils / materials (oil, petrol, gasoline, tar etc etc). I hope this helps. If not, then a good website to use is 'BBC bitesize, fractional distillation'. 

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