When is momentum conserved?

a. it is conserved when only two objects collide; more than two objects can't conserve momentum.

b. it is always conserved.

c. momentum is conserved when the system is closed, which means there are no forces, like friction, acting on it.

d. momentum is never conserved.

Two conductors, a and b, are each in the shape of a tetrahedron. but of different sizes. they are charged in the following manner: 1. tetrahedron a is charged from an electrostatic generator to charge q. 2. tetrahedron a is briefly touched to tetrahedron b. 3. steps 1 and 2 are repeated until the charge on tetrahedron b reaches a maximum value. if the charge on tetrahedron b was q/4 after the first time it touched tetrahedron a. what is the final charge qbmax on tetrahedron b?