Question 32·50 Super-Hard SAT Grammar Questions·Standard English Conventions

In historian Samira Qureshi combined, combined is the main verb, which says what the subject, Qureshi, did. Does the main part still need a verb? No, Qureshi already has her main verb, but the blank could belong to another clause, a group with its own subject and verb.

Which verbs belong to other clauses? which had been recorded according to several now-obsolete local calendars describes the dates, while before patterns among ports were compared tells when the dates were converted. Their verbs belong to those parts, and the blank sits outside both clauses. We can set them aside and look at what introduces the blank.

What job does with the dates give the blank? After with and a noun, the verb form only describes the noun, so this part adds a detail rather than a full sentence. The forms have been converted and are converted each give a main verb, which doesn't fit that structure. The dates are converted rather than converting something themselves, so the describing form must show that.

In with the dates, been converted lacks a helper: been needs a word such as have or having before it, and neither is there. Being converted uses the -ing form being with converted to show that the dates are changed. This lets it describe the dates without adding a main verb. Choice B.