Biology · Developmental Biology

In an experiment, activin-secreting beads were placed on unspecified cells from an early Xenopus embryo. The activin then diffused from the beads. If the beads contained 1 nM of activin, it elicited the expression of the Xbra gene in cells near the beads. If the beads contained 4 nM activin, the expression of Xbra was elicited in cells further away, but near the source bead, expression of the goosecoid gene was observed instead. Beads with no activin did not elicit the expression of either gene. Consider the following statements regarding these observations: A. High concentration of activin activates goosecoid, whereas lower concentrations activate Xbra. B. Lower concentrations of activin help specify the dorsal-most structures of the frog's embryo. C. Concentrations of activin below the threshold for these two genes specify cells to become blood vessels and heart. Which of the above statement(s) is/are correct?