Activin Is a Protein That Normally Exists in a Gradient

Activin is a protein that normally exists in a gradient within certain tissues in developing embryos. Certain undifferentiated cells cultured without activin produce epidermal cells. However, activin induces these same cells to adopt other fates (e.g., blood, muscle, heart) , depending on the concentration of activin. Based on these observations, activin acts as a(n) :

A) transcription factor.
B) cell-surface receptor.
C) morphogen.
D) second messenger.
E) activator.