For a basic \texttt{SELECT A, B FROM R} query, we scan the file and for each tuple output $A, B$.
Obviously, that means that for $N$ tuples and ratio of included tuples of $Q$, we have \cost{$N + N \cdot Q$} I/O operations.

For a \texttt{SELECT DISTINCT A, B FROM R} query, we scan the file and eliminate duplicates before outputting.
If this is also combined with a sort, we can prefer the sort-based approach and deduplicate after search,
so we are projecting, then sorting and finally deduplicating using the sort method.