A parabola declines from the left end and rises from the center through the right end of a broken horizontal line with a length of 22 centimeters. The height between the parabola and the horizontal line at the center is 0.5 centimeters. The diameter of a parabolic mirror is 22 centimeters, and the mirror has a depth of 0.5 centimeters at the center. What is the equation of the cross-sectional parabola of the mirror?
a. y = -x² / 11 + 0.5
b. y = x² / 11 - 0.5
c. y = -x² / 22 + 0.5
d. y = x² / 22 - 0.5