I think its a tough call on B. Though it serves the purpose. Argument mentions that reducing min wage will help not increase youth unemp. B mentions that even when wage are fixed the unemployment has risen, it means there is something different than wage that causes actual unemp rise.

1. Teenagers are unemployed because of the minimum wage
2. A subminimum wage for teens would stop the rising teenage unemployment rate from continuing to increase.

The question wants you to find a reason why the conclusion will no be valid

A. Incorrect. If unemployment has risen when wages rise it does mean there is a direct correlation between wages and employment. Thus a reduction should no longer increase the rate.

B. Correct. This calls the conclusion into doubt. The conclusion is based on the authors belief that the minimum wage is what is forcing people to not hire teens. If the wage is not rising but the unemployment rate still is then there must be another fact causing the rate to increase. If this is the case a subminimum wage may not stop the unemplyment rate from increasing.

C. Incorrect. This has no bearing on the conclusion.

D. Incorrect. We dont care about the rate of increase, we need to find a way to tie the rate in with the minimum wage.

E. Incorrect. This does not tell us why the rate declined or rose. Were there years when the decline was due to wage freezes? This doesnt give us the necessary info