The Methylation State of an Individual CpG Island

The methylation state of an individual CpG island:

A) is fixed; such genes are permanently turned off.
B) is fixed, but this has no effect on whether genes are expressed.
C) is random-sometimes the cytosines are methylated and sometimes they're not, but the state is independent of the environment or cell type.
D) can change over time in response to environmental cues, allowing genes to be turned on or off as needed.
E) can change over time in response to environmental cues, but this has no effect on gene expression.