Consider below statements – (I) Each non- leaf node in B+ tree has between [n/2] and n Children, where n is fixed for a particular tree. (II) The search key of a clustering index is always primary key. (III) Secondary indices must be dense. (IV) In a dense index, an index entry appears for only some of the search-key values. Which of the above statements are true?

- B+ Tree Structure: Non-leaf nodes in a B+ tree have a minimum and maximum number of children, dictated by the tree's order (`n`). This ensures balanced structure for efficient searching. - Clustering Index: Determines the physical order of data records. While often the primary key, it's not exclusively so. Other attributes can be used if they provide a logical clustering. - Dense Index: An index where an entry exists for every search-key value in the data file. This is mandatory for secondary indices to ensure all records are addressable. - Sparse Index: An index where entries exist only for some search-key values.