Video Explanation

Text Explanation

The idea in the second half of the sentence is that what appears as a force is something else.  In the first place, we need a contrast word, to mark the difference from the power expressed in the first half of the sentence.  The word "but" in choices (A) & (B) & (D) is best.   Choice (C) mechanically puts all the elements in the second half in parallel, but logically, they are not parallel element.  The aspect of appearing as a force and the aspect of simply being inertia are in sharp logical contrast, and putting these two in parallel obscures that contrast.   Choice (C) is incorrect. 

In (A), the use of the present progressive "is appearing" is unwarranted. This choice is incorrect.

Choice (B) jumbles the logic.  The original tells us that some that appears to be a force isn't really a force.  This choice suggests that "what appears to be a force" winds up obeying Conservation of Angular Momentum—a noble achievement for something that doesn't exist!! This choice is illogical and wrong.

In choice (E), the present perfect tense "have turned" is unusually and a bit awkward.  The big problem with (E) is the parallelism: the second "that" clause is presented in parallel to the first, but it doesn't modify the noun "apparent forces" as the first "that" clauses does.  Parallelism is a logical structure as well as a grammatical structure, and this choice creates completely illogical parallelism.  This is wrong. 

Choice (D) makes no mistakes and correct communicates the relationship of ideas in the second half.  Choice (D) is the best answer.