A +4μC point charge is located at x=0 and a +9μC point charge is at x=0.30m on the x-axis. At what position along the line joining the charges is the net electric field equal to zero?
a) x=0.15m from the +4μC charge
b) x=0.09m from the +4μC charge
c) x=0.12m from the +4μC charge
d) x=0.18m from the +4μC charge