Concept:The missing sentence should complete the idea that productivity is the key to economic growth.
Explanation:The paragraph explains that growth can come from more labour, more capital, or better use of both, known as total factor productivity (TFP).
It then says that TFP can come from brilliant innovations such as those of Thomas Edison.
The blank needs a concluding sentence that connects productivity to the recovery of economic growth.
"Whatever the reason" refers to the various sources of growth already mentioned in the passage.
The word "rebound" means to recover, and "perked up" means to improve.
So the sentence means that economic growth did not recover until productivity improved.
This fits the grammar and context of the paragraph perfectly.
Answer:Option C: "Whatever the reason, economic growth did not rebound until productivity perked up."