Yes, you do. In order to separate two independent clauses, you need to place a comma before the coordinating conjunction but
Michael A.
tutor
Yes, it is. The subject is the demonstrative pronoun that, and the predicate is the remainder of the clause. One way to check is to ask whether the clause can stand alone. "That is not the most likely scenario" can certainly stand alone as a sentence.
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10/12/16
Mark M.
10/12/16