This helps explain why those industries have productivity issues, but it leaves the role of computers unanswered. It doesn’t suggest that computers might actually help improve the situation.

This isn’t a useful correlation. For one thing, we don’t know whether these less industrialized nations were also increasing their reliance on computers. If they were, (B) would actually strengthen the argument, as it shows the drop in productivity growth is even more extensive.

Do these industries involve businesses that have increased their reliance on computer technology? And if so, what was the effect, if any, on productivity for those particular businesses that did? Without knowing this, we cannot say what effect (C) has on the argument.

This suggests that increased reliance on computers does improve productivity, and prevents the industry-wide trends in productivity losses from being even worse. It also challenges the assumption that what’s generally true of whole industries is true of any business.

This is about what might be true in the future, but the author’s conclusion is only interested in the situation to date. The promise that computers will probably improve productivity down the road says nothing about whether computers have managed to improve productivity to date.