In the question there is a key word: “largely”. I interpret the question to mean “Was the ad campaign responsible for most of the increase in October?” and specifically NOT to mean “Did the ad campaign have any positive effect on the sales?”.
The answer to the latter question is “very likely”, but to the former is merely “somewhat likely” because you have high variation in monthly revenues and just on the basis of data provided you cannot confidently assert that the difference between September and October was driven by the ad campaign.
In the question there is a key word: “largely”. I interpret the question to mean “Was the ad campaign responsible for most of the increase in October?” and specifically NOT to mean “Did the ad campaign have any positive effect on the sales?”.
The answer to the latter question is “very likely”, but to the former is merely “somewhat likely” because you have high variation in monthly revenues and just on the basis of data provided you cannot confidently assert that the difference between September and October was driven by the ad campaign.