Çözüldü Sentence Completion with Pronoun-antecedent and Tense Agreement - Punctuation

Manned flights to Mars are not planned the far-distant future ________ costly technology that is yet to be developed.
A) because it requires
B) because it would require
C) because they require
D) each requires
E) as they are requiring

Correct Choice: C

A) Pronoun-antecedent inconsistency. "Flights" is plural; "it" is singular.
B) Same as A
D) Run-on sentence. Punctuation is needed between "future" and "each" in order to separate the two independent clauses
E) Improper usage of progressive tense. The sentence refers to events that will occur in the future. The verb "are requiring" pertains to the present.

Source: Barron's SAT II Writing, 3rd Ed., 2001, page 355, Question 29, explanations (slightly different) on page 367