Consider two charged spheres, one with charge $+2$ C and the other with $2$ C$.$ A proton $($a positively charged particle$)$ is at the point halfway between the spheres. What is not zero?

a$.$

the potential energy of the proton

b$.$

the work to move the proton from infinity to that point

c$.$

the force on the proton

d$.$

all of the three above are zero