Which of the following is a good example of a feedback loop in machine learning?

A shopping app surfaces new items to buy, which is based on dataset from customers fitting a similar profile. When you buy, you go into that dataset

A social media site surfaces controversial posts, which make users more angry and lead to more angry posts on the network

A social media site tracks engagement, uses an algorithm to surface posts you're likely to engage with, which then goes back into the algorithm

A shopping app tracks your purchases, and recommends new things to buy