user: Question: WHAT IS THE SOURCE WITH A TYPE OF Transfer AND MEX?
Columns: ['Nat.', 'Name', 'Moving to', 'Type', 'Transfer window', 'Transfer fee', 'Source']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['BRA', 'Edmílson', 'Villarreal', 'Contract Termination', 'Summer', 'Free', 'Barcelona.cat']
SQL:

assistant: SELECT "Source" FROM "Table" WHERE "Type" = 'Transfer' AND "Nat." = 'MEX';