Question $7$

Which of the following is$/$are true about collaborative filtering?

a$.$User$-$based similarity is usually more stable than item$-$based similarity.

b$.$If you have a dataset of user ratings on some products, you can use these to predict one user's preferences on products that he has not rated.

c$.$To use collaborative filtering, you need to manually design a set of features for every item $($e$.$g$.,$ movie$)$ in your dataset, which describes item's most important characteristics

d$.$Amazon's item$-$based collaborative filtering requires pre$-$processing.