Sentence Correction (SC) | Re: Digging in sediments in northern China, evidence has been gathered by

mikemcgarry

how is the phrase "Than previously thought" clear? Shouldn't it have been "than they had previously thought"? Because from what I understand, GMAT wants you to be clear when it comes to the subject of the phrase.

Please correct me if there's anything that I am not thinking about correctly.

---