The graph of the function $y = f(x) + 21$ can be obtained from the graph of $y = f(x)$ by one of the following actions:
(a) shifting the graph of $f(x)$ to the right 21 units;
(b) shifting the graph of $f(x)$ to the left 21 units;
(c) shifting the graph of $f(x)$ upward 21 units;
(d) shifting the graph of $f(x)$ downward 21 units;
Your answer is (input a, b, c, or d)