D.        None of the Above
In the early stages of the automobile there were a number of alternative fuels considered and used. As
a matter of fact, all fuels at the time would have been considered 'alternative'...Aside from the 'wheel'
there was no existing precedent applicable to most phases and features of this new technology.
If to behold only the early stages of the automobile most would not predict that fossil fuels would soon
become the near-universal commodity employed to propel ever-increasing numbers.  It would seem
inconceivable this fuel source would continue to dominate for generations to come...
But that it did...

By the turn of the 21st century the automobile had evolved to represent a rather impressive 'feat of
engineering''.  Technology and engineering brought sophisticated safety features controlled and
regulated by numerous microcomputers- all of which communicated with the other.  Advanced
communication devices, satellite navigation systems, climate control,  were but a few among many other
features,

And yet, after all these advances?...  They still relied upon a fuel source with a number of notorious
properties...

First,  the conversion rate to mechanical energy is/was very poor (25% at best)...It was a combustion
engine after all!  Heat, friction, and idling account for the remainder... It is aimless energy doing good by
no one (other then perhaps to warm things up a little bit)

Second, Fossil fuels are/were filthy, finite and toxic

Third, fossil fuels were difficult to extract, process, and transport- a situation that got only worse as
supplies begin to dwindle.

It was fossil fuel used in the extraction of fossil fuels-  the monster consuming its own tail- rapidly
becoming harder and harder to reach.
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COMBUSTION
(Expanded)
4.  In what Decade was the combustion engine no longer available for purchase?
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Forensics of Destructive Practice

For generally obvious reasons the combustion engine was phased-out long ago.  It is now regarded as
a just a curiosity in select museums.  The issue not so clear is,
why did it take so long?.    

For  reasons, seeming without explanation, the combustion engine was used in propulsion with near
exclusivity for decades after alternative technologies were readily available.  Until recently, the answer to
this query had fully eluded historians...