What Does the "Clock and Wavefront" Model Propose

What does the "clock and wavefront" model propose?

A) Cells must undergo a specific number of oscillations of gene activity in order to form a somite
B) Cells must be passed by the wavefront to commit to forming a somite
C) Cells must not have yet been passed by the wavefront to commit to forming a somite
D) Genes that position a wavefront have clock-like gene expression