Histone Modification

Histone modification:

A) 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.
B) is fixed, but this has no effect on whether genes are expressed.
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) 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.