Numerous paintings and engravings representing warfare can be found in remains of all civilizations going back to and including the Neolithic period, when agriculture was first developed. However, no paintings or engravings of warfare are found dating from before the Neolithic period. Ttherefore, warfare must have first developed as a result of the transition to an agricultural society. Which one of the following is an assumption required by the argument? ['Warfare in the Neolithic period was always motivated by territorial disputes over agricultural land.', 'Paintings and engravings of agricultural life began to be made at the same time as paintings and engravings of warfare.', 'Warfare is the inevitable result of the development of a civilization.', 'There was no warfare prior to the period in which paintings and engravings of warfare were first created.']