When the first law of thermodynamics (U = Q + W) is applied to a system S,

When the first law of thermodynamics (ΔU = Q + W) is applied to a system S, the variables Q and W stand for
(a) The heat flow out of S and the work done on S.
(b) The heat flow out of S and the work done by S.
(c) The heat flow into S and the work done by S.
(d) The heat flow into S and the work done on S.