Let f(x)=kx+7, where k is a real constant. Which one of the following describes the monotonic behaviour of f on R?
a) f is strictly increasing if k≥0 and strictly decreasing if k<0.
b) f is strictly increasing if k<0 and strictly decreasing if k>0.
c) f is strictly increasing if k>0 and strictly decreasing if k<0.
d) f is strictly increasing for every k=0.