Help Detective Barnaby complete his case notes about the messy kitchen. Choose the correct phrase to complete the sentence.

The kitchen was covered in flour, and the oven was still warm. It was obvious that someone ___ cookies before I arrived.

The correct answer is had been baking.

We use the past perfect continuous (had been + verb-ing) to show that an ongoing action caused a visible result in the past. The flour and the warm oven were the past evidence of the ongoing baking! "Has been baking" is incorrect because the entire situation is in the past, not the present.