Histone modification:
a. Can change over time in response to environmental cues, allowing genes to be turned on or off as needed.
b. Is fixed; once a histone is modified, it stays that way and the genes with which it is associated are turned on or off permanently.
c. Is random; sometimes the lysines are modified and sometimes they're not, but the state is independent of the environment or cell type.
d. Is fixed, but this has no effect on whether genes are expressed.
e. Can change over time in response to environmental cues, but this has no effect on gene expression.