"Only" is a group two necessary indicator. But "THE Only" is a group one sufficient indicator. Definitely tricky

P1) If you are a rat then you like cheese

P2) Remy is a rat

Concl) therefore, Remy likes cheese

Since the stimulus says "Blackouts will probably occur, unless heat wave abates" it shows that the blackouts are caused by the ACs. The "unless heat wave abates" is especially important because it assumes that if the heat wave stops, people will then stop using their ACs as much, thus causing an end to the power outages. This question is tricky because it is this vague connection that proves the ACs cause the outage.