Light passes successively through two polarimeters tubes each of length `0.29m`. The first tube contains dextro rotatory solution of concentration `60kgm^-3` and specific rotation `0.01 rad m^2kg^-1`. The second tube contains laevo rotatory solution of concentration `30kg//m^3` and specific rotation `0.02 rad m^2kg^-1`. The net rotation produced is
A. (a) `15^@`
B. (b) `0^@`
C. (c) `20^@`
D. (d) `10^@`
