Only programs 1 and 2 will create a list of unique Customer_ID values from the WORK.ORDERS data set. Program 1 uses the NODUPKEY option in PROC SORT to remove duplicates when sorting by Customer_ID. Program 2 uses PROC SQL with the DISTINCT keyword to select unique Customer_ID values. Program 3 will not work correctly because it relies on the FIRST. variable, which is set in a BY group processing step that requires the data to be sorted by Customer_ID first. Since the data is initially unsorted, program 3 will not produce the desired result.