Consider the function f(x)=21−x+3. Which option correctly lists its horizontal asymptote and range?
a) Horizontal asymptote y=0, range (0,∞)
b) Horizontal asymptote x=0, range (3,∞)
c) Horizontal asymptote y=3, range (3,∞)
d) Horizontal asymptote y=3, range (−∞,3)