From the Shareholder's Point of View, the Optimum Time to Call

From the shareholder's point of view, the optimum time to call a convertible bond is when the bond's conversion value is:

A) less than the call price, but greater than the face value.
B) greater than the call price, but less than straight debt's value.
C) equal to the face value.
D) less than straight debt's value, but greater than the call price.
E) none of the above.