The Figure Above Represents the Production Possibilities Frontier for a Country

The figure above represents the production possibilities frontier for a country.
a) The nation is currently producing at point B and wants to move to point C. What is the opportunity cost of the move?
b) The nation is currently producing at point B and wants to move to point A. What is the opportunity cost of the move? $$\begin{array} { | l | l | l | l | } \hline \begin{array} { l } \text { Production } \\\text { point }\end{array} & \begin{array} { l } \text { Beef } \\\text { (pounds) }\end{array} & &\begin{array} { l } \text { Wheat } \\\text { (bushels) }\end{array} \\\hline A & 0 & \text { and } & 9 \\\hline B & 2 & \text { and } & 7 \\\hline C & 4 & \text { and } & 4 \\\hline D & 6 & \text { and } & 0 \\\hline\end{array}$$