Consider the three choices (i), (ii), and (iii) to complete the sentence below. Select the answer that contains all of the correct choices but no incorrect choices. The momentum of a system is constant
(i) if there is no net external force
(ii) if there are no external forces (the system is isolated)
(iii) always


o (i) only
o (i) and (ii) only
o (ii) only
o (i), (ii), and (iii)