Sentence C uses an absolute phrase to add variety and interest. The absolute phrase is the NP "feet tapping". An absolute phrase is a phrase that modifies a noun in a sentence, but it is not connected to the sentence by a conjunction. It is set off with a comma and could be removed from the sentence without altering its meaning (see 1).
1) The principal waited to hear an explanation for how thirty desks ended up on the cafeteria roof last night
