We want to create a Firestore collection as follows: Collection to be called rooms and for every document in rooms, there should be a subcollection called messages. Here's schema that we wish to achieve:

Firestore Subcollections image

Which of the following statements are true? A: Collections cannot exist inside documents B: Collections can exist inside documents as subcollections C: When a document is deleted, subcollections within the document are automatically deleted D: When a document is deleted, subcollections are not deleted and has to be deleted manually