In a region of space,a uniform magnetic field $B$ exists in the $y-$direction. $A$ proton is fired from the origin,with its initial velocity $v$ making a small angle $\alpha$ with the $y-$direction in the $yz$ plane. In the subsequent motion of the proton,

• A
  its $x-$coordinate can never be positive
• B
  its $x-$ and $z-$ coordinates cannot both be zero at the same time
• C
  its $z-$ coordinate can never be negative
• D
  its $y-$ coordinate will be proportional to the square of its time of flight