A particle of mass $m$ and charge $q$ experiences a force $\vec{F}_e$ in an electric field $\vec{E}$ and it is directed along the field. When the particle is moving with velocity $\vec{v}$ in a magnetic field $\vec{B}$, it experiences a force $\vec{F}_B$, that is perpendicular to both $\vec{v}$ and $\vec{B}$. In presence of both fields, the force acting on the particle is the vector sum of that due to $\vec{E}$ and that due to $\vec{B}$. A current-carrying conductor also experiences a force in presence of a magnetic field. The force $d\vec{F}$ acting on a current-element $I d\vec{l}$ is perpendicular to both the current element, and the magnetic field $\vec{B}$. The net force acting on a conductor of a given length is the vector sum of all $d\vec{F}\text{s}$. (i) A negative ion enters a region of uniform magnetic field $\vec{B}$ as shown in the figure. It will : (A) go straight, undeviated (B) describe a circular path (C) describe a semicircular path (D) describe a helical path