What has been done to avoid sources of bias in Dr. Rablen and Dr. Oswalds study？A．Excludin

What has been done to avoid sources of bias in Dr. Rablen and Dr. Oswalds study？

A．Excluding the abnormal deaths.

B．Selecting more prize nominees than winners.

C．Having all subjects from the same country.

D．Assuming inflation as an irrelevant factor.

请帮忙给出正确答案和分析，谢谢！