The displacement of a body is given by `r=sqrt(a^(2)-t^(2))+t cost^(2)` , where `t` is the time and a is constant. Its velosity is `:`
A. `(-t)/(sqrt(a^(2)-t^(2)))+cos t^(2)-t sin 2t`
B. `(-t)/(sqrt(a^(2)-t^(2)))+cos t^(2)-2t^(2) sin 2t^(2)`
C. `(-a)/((a^(2)-t^(2)))+2 t cos t^(2) sin t+sin t`
D. `a -t^(2)- t sin t^(2)`