Help! External Sort/Merge Algorithm Needed

External sort/merge algoritm used for sorting large data files that
the main memory of the computer.
In the sorting stage, the records are read from the input(unsorted) file
sorted into several paritions which are written into the output file.
Each parition is
sorted in order.
There are 3 possible methods used in the sorting stage: internal
replacment selection & natural selection algorithm.
Can anybody help me in the algorithms of the 3 methods ?