英语翻译Only in the most exceptional circumstances is the lifesp

英语翻译
Only in the most exceptional circumstances is the lifespan of a patent extended to alter this normal process of events.
The longest extension ever granted was to Georges Valensi; his 1939 patent for color TV receiver circuitry was extended until 1971 because for most of the patent’s normal life there was no colour TV to receive and thus no hope of reward for the invention.
43.George Valensi’s patent lasted until 1971 because ________.
[A] nobody would offer any reward for his patent prior to that time
[B] his patent could not be put to use for an unusually long time
[C] there were not enough TV stations to provide colour programmes
[D] the colour TV receiver was not available until that time
我选了D 能不能帮我分析一下