<p>Until relatively recently humans were thought to be uniquely self-aware, (scientists now know) that most chimpanzees and orangutans can recognize their own reflections.
(b) but scientists now know
(c) however, scientists now know</p>
<p>The answer is b.
Why is original sentence wrong? Isn't that until work as a conjunction?</p>
<p>Although Daniel Gabriel Fahrenheit was far from (being) the (only) eighteenth-century scientist (to propose) a temperature scale, (but his) was no of the few that were widely used.
The answer is (but his).
Why is that wrong. Isn't that it refer to his temperature scale?</p>