SAT Form, Structure, and Sense Question
Recent pollen analyses of the Aran Islands have led some researchers to propose that the now treeless islands were once wooded. This hypothesis that certain trees, such as P. sylvestris, survived without interruption or human intervention throughout the Holocene cannot stand, researchers Michael O'Connell and Karen Molloy counter, unless other explanations can first be ruled out. Which choice completes the text so that it conforms to the conventions of Standard English?
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The trap here
SAT verb forms: when the blank needs -ing and when it needs a real verb — Choosing between "combines", "combining" and "to combine" is not about tense. It is about whether the sentence already has a main verb.