Ten E.coli cells with `` { }^{15} N - dsDNA `` are incubated in medium containing `` { }^{14} N `` nucleotide. After `` 60 `` minutes, how many E.coli cells will have DNA totally free from `` ^{15}N `` ?
(A)80 cells
(B)20 cells
(C)40 cells

(D)60 cells