Two particles, each of mass 7.0 kg, are a distance 3.0 m apart. To bring a third particle, with mass 21 kg, from far away to a resting point midway between the two particles, an external agent must do work equal to
A. 1.3 x 10-8 J
B. –4.4 x 10-9 J
C. –1.3 x 10-8 J
D. –6.5 x 10-9 J
E. 6.5 x 10-9 J
