Identify the comma rule that the following sentence breaks:

in their homeroom classes, students learned of a new very strict rule that would affect their parking privileges.

a) use a comma to separate three or more items in a series.
b) use a comma to set off a phrase or clause at the beginning of a sentence.
c) use a comma to separate coordinate adjectives.
d) use a comma to set off a parenthetical element in the sentence.