Mark the *false* statement with regards to the document frequency (df)?
1) Frequent terms are less informative than rare terms.
2) The 'df' of a term 't' can be found as the length of the posting list of 't'.
3) The 'df' is a direct and proportional measure of the informativeness of 't'.
4) The 'df' of a term 't' refers to the number of documents that contain 't'.