"Tobacco Plants Are Grown in Soils with High-Phosphate Fertilizers That $\alpha$

"Tobacco plants are grown in soils with high-phosphate fertilizers that are naturally contaminated with the alpha-particle emitting radionuclide polonium-210.In one year,the average smoker will irradiate the bronchial epithelium with 8 to 9 rem,the equivalent dose of radiation from 250 to 300 chest X-ray films per year" (Pediatrics,September 1993,p.464) .How many grays is equivalent to 8.5 rem for $\alpha$ particle emission? (RBE for $\alpha$ particles = 20)

A) 43 Gy
B) 0.0043 Gy
C) 1.7 Gy
D) 850 Gy
E) 0.085 Gy