Which of the Following Is Not an Explanation Put Forward

Which of the following is not an explanation put forward to explain the recent productivity slowdown?

A) Consumers and firms are paying down high levels of debt instead of investing in productive projects.
B) Overinvestment during the boom that preceded the Great Recession created overcapacity.
C) Technological progress itself may have stalled, until invention and innovation levels bounce back.
D) A massive influx of immigrant labor has caused the labor force to expand much faster than real GDP.