Four charges are placed at the four corners of a square of side 15 cm. the charges on the upper left and right corners are +3 μc and -6 μc respectively. the charges on the lower left and right corners are -2.4 μc and -9 μc respectively. the net electric force on -9 μc charge is: a. 18 n, 750 below the positive x-axisb. 18 n, 750 below the negative x- axisc. 18 n, 750 above the negative x- axisd. 18 n, 750 above the positive x- axis