GMAT Critical Reasoning (CR) | Re: Cotrell is, at best, able to write magazine articles of average quali

Hi Expert,

I have a difficult time in identifying the conclusion of this argument. I assumed that sentence--"Cotrell is, at best, able to write magazine articles of average quality" is a premise, and the sentence --"he must obviously have plagiarized superior ones." is a conclusion. Moreover, below sequence of premise-conclusion also fits perfectly

Cotrell is, at best, able to write magazine articles of average quality, so he must obviously have plagiarized superior ones.

Please