Theorists of expeditions to extrasolar planets have typically been concerned with exceeding the speed of light and to what degree it could be, but aerospace engineers have long expected that the greatest obstacles to the manned exploration of other solar systems would be the difficulty of prolonged food storage and exposure to cosmic radiation.
A) Theorists of expeditions to extrasolar planets have typically been concerned with
exceeding the speed of light and to what degree it could be
B) Theorists of expeditions to extrasolar planets have typically had as their concern the
exceeding of the speed of light and to what degree it could be
C) Theorists of expeditions to extrasolar planets have typically been concerned with whether
the speed of light can be exceeded and to what degree - CorrectD) The theorists of expeditions to extrasolar planets have typically been concerned with the possibility of
exceeding the speed of light and to what degree it would
E) The theorists of extrasolar expeditions to other planets have typically been concerned with whether the speed of light can be exceeded and the degree
that is
Theorists are concerned whether the speed of light can be exceeded not that they can exceed the speed of light (which is what "exceeding the speed of light" tries to convey) - A, B and D - Eliminated
In E) that has no antecedent
C) communicates it properly using to what degree it would exceed if the speed of light can be exceeded