6. Consider a point particle of mass m lying at a distance d above some infinitely large sheet, whose mass per unit area is σ. What is the magnitude of gravitational attractive force between these two objects? (G is the gravitational constant.) (A) 4πGmσ (B) 2πGmσ (C) Gmσ/2 (D) Gmσ/d² (E) Gm²/(4d²).