Two point charges lie on the x axis. A charge of +2.50pC is at the origin, and a charge of -4.80pC is at x=-12.0cm. Part A What third charge should be placed at x=+24cm so that the total electric fie
a) +1.20 pC
b) -1.20 pC
c) +2.40 pC
d) -2.40 pC