GMAT Sentence Correction (SC) | Re: Authors such as Elizabeth Kolbert were beginning to warn tha

Authors such as Elizabeth Kolbert were beginning to warn that human activity — greenhouse-gas emissions, urbanization, the global spread of invasive species —were driving Earth toward a “mass extinction” event and something that had occurred only five times since life emerged over 3 billion years ago.

A were driving Earth toward a “mass extinction” event and something that had occurred only five times since life emerged
B were driving Earth toward a “mass extinction” event,
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