Which of the following process are thermodynamically irreversible?
1. Gradual heating of water from a hot source
2. Isothermal expansion in a Stirling cycle
3. Mixing of two ideal gases at constant pressure
(A) 1, 2 and 3 (B) 1 and 2 only
(C) 2 and 3 only (D) 1and 3 only