Let `alpha` and `beta` be the roots of the equation `x^2+(2-costheta)x-(1+costheta)=0`, then
A. least value of `alpha^2 + beta^2` is 5
B. `alpha^2+beta^2` is least when `theta=2npi,n in I`
C. least value of `alpha^2+beta^2` is 6
D. `alpha^2+beta^2` is least when `theta=(2n+1)pi/2, n in I`
