Two massless bags contain identical bricks, each brick having a mass M . Initially, each bag contains four bricks, and the bags mutually exert a gravitational attraction F1 on each other. You now take three bricks from one bag and add them to the other bag, causing the bags to attract each other with a force F2. What is the closest expression for F2 in terms of F1?

a. F2 =3/4F1.
b. F2 = 1/2F1.
c. F2= 1/4F1.
d. F2 = F1 because neither the total mass nor the distance between the bags has changed.