Suppose you manage a wine cellar with fixed storage space and you have to choose when to sell each bottle of wine. The value of each bottle V(t) is increasing over time at a decreasing rate: V' (t) > 0 and V'' (t) < 0. Once you open a bottle you free up storage space for a fresh bottle with value V(0). The optimal time to sell each bottle is defined by what condition? Explain your answer.