A charged particle moves with velocity `vec v = a hat i + d hat j` in a magnetic field `vec B = A hat i + D hat j.` The force acting on the particle has magnitude F. Then,
A. `F=0, if aD=dA`.
B. `F=0, if aD=-dA`.
C. `F=0, if aA=-dD`.
D. `fprop (a^2+b^2)^(1//2)xx(A^2+D^2)^(1//2)`
