When performing a select for a value that is in an index

When performing a select for a value that is in an index

A.
the row may be used exclusively to return the data

B.
the index may be used exclusively to return the data

C.
both the row and the index are always used to return the data

Explanation:
MyISAM engine can determine which values satisfy the query from the index file alone, and return data withour reading data file at all.