Liquid helium（atomic weight 4.003） has a density r= 0.13 g/cm3.Estimate the radius of a He atom， assuming that the atoms are packed in the densest possible configuration， which fills 74% of the space.（）。

A.1.3 nm

B.0.5 nm

C.134.7 nm

D.20.8 nm

正确答案：20.8 nm