The first law of thermodynamics states that the change U in the internal energy of a system

The first law of thermodynamics states that the change ΔU in the internal energy of a system is given by ΔU = Q − W, where Q is the heat and W is the work. Both Q and W can be positive or negative numbers. Q is a positive number if ________, and W is a positive number if ________.

(a) The system loses heat; work is done by the system

(b) the system loses heat; work is done on the system

(c) The system gains heat; work is done by the system

(d) The system gains heat; work is done on the system