A thick metal slab carries a current I as shown. A uniform magnetic field B points perpendicular to the slab (and into the page). If point P on the slab is at a higher potential than point Q , what can we say about the charge carriers that produce the current in the slab?

Question

A thick metal slab carries a current I  as shown. A uniform magnetic field B  points perpendicular to the slab (and into the page). If point P  on the slab is at a higher potential than point Q , what can we say about the charge carriers that produce the current in the slab?
a.They are positive. 
b. They are negative.  
c.They could be either positive or negative, but we can't tell without more information.  
d.The situation is impossible: point   could never be at a higher potential than point  , no matter what the sign of the charge carrier.