A potentiometer is used for the comparison of e.m.f. of two cells \[{{E}_{1}}\] and \[{{E}_{2}}\]. For cell \[{{E}_{1}}\] the no deflection point is obtained at 20\[cm\] and for \[{{E}_{2}}\] the no deflection point is obtained at 30\[cm\]. The ratio of their e.m.f.'s will be [MP PET 1984]

• A.                    2/3
• B.                                      1/2
• C.                    1
• D.                                      2