(C) is the only possible answer to the question.
(A) and (E) both contain the unsupported pronoun "they". Every pronoun on the GMAT will have an antecedent (the only exception to this rule is "it", which can appear in reference to the universe as a whole, e.g. "it is a nice day today").
We can also eliminate (B) for verb tense problems. "as" indicates that things are going on simultaneously, so switching to "had just been" is wrong.
(D) has
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(A) and (E) both contain the unsupported pronoun "they". Every pronoun on the GMAT will have an antecedent (the only exception to this rule is "it", which can appear in reference to the universe as a whole, e.g. "it is a nice day today").
We can also eliminate (B) for verb tense problems. "as" indicates that things are going on simultaneously, so switching to "had just been" is wrong.
(D) has
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