[单选题]
An object of mass $m$ and charge $q$ is dropped vertically between two parallel plates, $K$ and $L$. A magnetic field $B$ can be created perpendicular to the plane of the paper. The electric field between plates is $E$.
Which of the following is correct about the magnitude of magnetic and electric fields when the object falls vertically at a constant velocity $v$?
A. $E$ is $\frac{mg}{q}$ ; $B$ is $\frac{mg}{qv}$
B. $E$ is mv ; $B$ is 0
C. $E$ is mg ; $B$ is $\frac{m}{qv}$
D. $E$ is $\frac{mg}{q}$ ; $B$ is 0
