small spheres a and b are attached by a 20cm long string. the charges on spheres a and b are 2.4μc and 3.6μc, respectively. the tension in the string holding them together is most nearly 0.019n 0.019 newton 0.39n 0.39 newton 1.30n 1.30 newtons 1.94n 1.94 newtons 2.92n