What observation did Dr. Katsusaburo Yamagiwa make to lead him to believe that chemicals cause cancer?
A. He observed that cancer rates were 40 times higher in smokers than in nonsmokers.
B. He observed that when coal tar extracts were applied to rabbit skin, cancers eventually developed.
C. He observed that chimney sweeps exhibited frequent cancer of the scrotum.
D. He observed unusual tumors of the nose in snuff users and suggested tobacco as the cause.
Answer: B
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