Potential Energy of Point-Charges: Two Point Charges of +1

Potential energy of point-charges: Two point charges of +1.0 μC and -2.0 μC are located 0.50 m apart. What is the minimum amount of work needed to move the charges apart to double the distance between them? (k = 1/4πε₀ = 8.99 × 10⁹ N ∙ m²/C²)

A) -36 mJ
B) +18 mJ
C) 0 mJ
D) +36 mJ
E) -18 mJ