Original post needs rectification. Here's the actual from Manhattan GRE:

Use of an augur has steadily declined as scientific advancement has made such use less necessary. Therefore, the postulate that science will eventually (i)__________the use of augurs is (ii)__________.

Blank (i)	Blank (ii)
annihilate	specious
consummate	plausible
supplant	expeditious

Explanation
An augur is a “soothsayer, a prophet.” This argument claims that use of soothsayers is declining. The word “eventually” in the second sentence suggests that the trend established in the first sentence will continue: science will eventually supersede the use of augurs. In blank (i), look for a word that means “supersede.”

(A) Annihilate means “to destroy something, to defeat somebody easily and decisively.” This has as the correct negative spin we want, but it is too strong.
(B) Consummate means “to fulfill or make perfect.” This is the opposite of what we what.
(C) CORRECT. Supplant means “to take the place of, especially something inferior.” This is a perfect match, especially because the argument suggests with the phrase “has made such use less necessary” that the rise of science caused a decline in the use of augurs.

The word “therefore” at the beginning of the second sentence suggests that the second sentence is a logical conclusion drawn from the first sentence. There is no pivot word. Therefore, the second sentence should agree with the first sentence. As we have filled blank (i) with a word that agrees with the first sentence, blank (ii) should have a positive spin, such as “believable.”

(D) Specious means “falsely appearing to be true.” This is the opposite of what we want. Perhaps this word would make sense if the postulate were that science will “bring back” the use of augurs. However, none of the answer choices for blank (i) would be logically followed by this claim in blank (ii).
(E) CORRECT. Plausible means “believable, appearing likely to be true.” This has a positive spin and fits nicely.
(F) Expeditious means “prompt or quick.” This does not fit.

"Use of an augur has steadily declined as scientific advancement has made such use less necessary"
This means that scientific advancement was probably replacing the function of "Augur" and thus augur was being demanded less.
Thus first blank goes with supplant.
second blank goes with plausible. Expeditious is wrong because first part of sentence said "steadily" means gradually not hurriedly .