Choose the graph of y = 2 cos x

Y = cos x begins at (0,1), descends to (pi/2,0), descends to (pi,-1), ascends to (3pi/2,0), and then ascends to (2pi, 1). This covers only one full period.

The graph of y = 2cost x is the same, except that the amplitudes (y-values) are 2x as great as before:

(0,2), (pi/2, 0), and so on.