Two point charges +q and +4q are fixed 10 cm apart on a straight line in air. For a small positive test charge, where along this line can the net electrostatic force be zero?
a) At the midpoint (5 cm) between the two charges
b) Between the two charges, about 3.3cm from the +q charge
c) Between the two charges, about 2.0cm from the +q charge
d) On the line outside the two charges, to the side of the +q charge