Critical Reasoning

American attitudes tend to be rather insular, but there is much we can learn from other countries. In Japan, for example, workers set aside some time each day to exercise, and many corporations provide elaborate exercise facilities for their employees. Few American corporations have such exercise programs. Studies have shown that the Japanese worker is more productive than the American worker. Thus it must be concluded that the productivity of American workers will lag behind their Japanese counterparts, until mandatory exercise programs are introduced.

The conclusion of the argument is valid if which one of the following is assumed?

(A) Even if exercise programs do not increase productivity, they will improve the American worker's health.
(B) The productivity of all workers can be increased by exercise.
(C) Exercise is an essential factor in the Japanese worker's superior productivity.
(D) American workers can adapt to the longer Japanese work week.
(E) American corporations don't have the funds to build elaborate exercise facilities.

OA C
source: LSAT prep book
I don't understand how the answer isn't B, because if that answer isn't valid (applied with negation test) then conclusion isn't valid too.

experts please help me to solve this question