CFA三级Alternative Investemnt中Commodity的问题

教材中对于远期价格低于现货价格的解释是，有隐含期权。详见下：Real options under uncertainty. Oil futures markets are often backwardated; in these markets, futures prices are                              often below the current spot price. This situation may be caused by the existence                              of real options under uncertainty.63 A real option is an option involving decisions related to tangible assets or processes. In other                              words, producers are holding valuable real options—options to produce or not to produce—and                              will not exercise an option to produce unless spot prices start to climb. Production                              occurs only if discounted futures prices are below spot prices, and backwardation results if the riskiness of future prices is sufficiently high. A major consequence                              of a downward-sloping term structure of futures prices is the opportunity to capture                              a positive roll return as investment in expiring contracts is moved to cheaper new                              outstanding contracts.

为什么卖方只有在远期价格低于现货价格的情况下才会生产呢？

求解释~