____ is the gravitational force of attraction between two spheres each of mass 90 kg if their distance from the center is 40 cm.
a) 6.67 x 10⁻11 N
b) 6.67 x 10⁻9 N
c) 9.8 x 10⁻11 N
d) 9.8 x 10⁻9 N