In the Fourth Step in Olefin Metathesis the ____

In the fourth step in olefin metathesis the ____.

A) metal alkylidene ([M]=) undergoes a [2+2] pericyclic addition with an alkene alkene to form a metalcyclobutane intermediate
B) metalcyclobutane undergoes a reverse [2+2] addition (i.e.,retro [2+2] reaction) to generate a new metal alkylidene
C) reaction of the new metal alkylidene with a new alkene (by [2+2] addition) to generate a new metalcyclobutane intermediate
D) formation of the alkene product from a retro [2+2] reaction of the second metal alkylidene
E) alkene reacts with a second alkene by [2+2] addition