Two particles, each of mass m, are a distance d apart. To bring a third particle, with mass 2m, from far away to a resting point midway between the two particles the work done by an external agent is given by: A. 4Gm2/d B. -4Gm2/d. C. 8Gm2/d2 D. -8Gm2/d2 E. zero ​