As a customer becomes more comfortable using computers, thei

Official explanation:
In the original sentence, the plural pronoun "their" refers to the singular noun "a customer." Hence, (A) is incorrect.

A vertical scan shows that all the choices except (C) end with "their," while (C) ends in the singular "his or her." (A) and (C) have singular subjects, while (B), (D), and (E) have plural subjects. Because the subject and pronoun agree in number in each choice except (A), which you've already eliminated, proceed to check each answer choice individually.

(B) keeps "their" but changes its antecedent to the plural "customers," so the noun and pronoun are now in agreement. (B) does not seem to introduce any other errors. Check the other choices to be sure this one is correct.

(C) and (D) can be eliminated because they incorrectly use the infinitive "to use." Eliminate (E) because it tries to use the gerund "becoming" as a main verb without a helping verb (e.g., "are becoming").

Only (B), the correct answer, has pronoun antecedent agreement and correct verb tenses.

TAKEAWAY: On the GMAT, pronouns must agree in number with their antecedents.