Which of the processes described below are irreversible ?    a)    The increase in temperature of an iron rod by hammering it        b)    A gas in a small container at a temperature T1 is brought in contact with a big reservoir at a higher temperature T2 which increases the temperature of the gas    c)    A quasi-static isothermal expansion of an ideal gas in cylinder fitted with a frictionless piston    d)    An ideal gas is enclosed in a piston cylinder arrangement with adiabatic walls. A weight W is added to the piston, resulting in compression of gas