The 11-Year Sunspot Cycle on the Sun Is

The 11-year sunspot cycle on the Sun is:

A) the regular movement of a relatively constant number of sunspots from the poles to the equator of the Sun over an 11-year period.
B) an irregular, sometimes absent variation in the number of visible sunspots.
C) a perfectly regular buildup and decay of the number of sunspots, with a precise period of 11.1 years.
D) a somewhat irregular but always present cycle of buildup and decay of sunspot numbers.