Which of the following file organizations is/are I/O efficient for the scan operation in DBMS?

A scan operation in a database retrieves all data records/pages of a relation:

• (A) Sorted file: Records are ordered and stored compactly across contiguous data blocks. Scanning reads each block sequentially, requiring only $B$ block I/Os (where $B$ is the number of data blocks), which is optimal.

• (B) Heap file: Records are packed in data pages without any particular order. A full scan simply reads every block of the file sequentially from start to end, which also costs $B$ block I/Os and is highly I/O efficient.

• (C) & (D) Unclustered tree/hash index: Because the index is unclustered, the physical ordering of records in the data blocks does not correspond to the index key order. Traversing the unclustered index to retrieve all records involves fetching records across arbitrarily scattered pages, potentially requiring up to $1$ I/O per record ($R$ I/Os, where $R \gg B$), which is extremely I/O inefficient for a full scan.