C tells us that productivity growth in industries related to computers have grown as computers became more widespread.
The argument we are trying to weaken is that productivity as a whole has not risen as a result of computers, so any individual business that has increased its use of computers has also not necessarily grown.
C shows that in a few instances the rise of computing has led to a rise in productivity, but that does not actually weaken the argument. The argument to is trying to prove that there is not a clear connection between productivity growth and tech- a few counterexamples do not really undermine that argument. The fact that computers led to growth in productivity in companies that made computers should not be a surprise. However, the way that computers effective productivity in companies who make computers (e.g. greater demand for product) is likely quite different from how the growth of computers impact other companies (changing how they do business by using more computers).