The introduction of symbols for numbers is an event lost in prehistory, but the earliest known number symbols, in the form of simple grooves and scratches on bones and stones, date back 20, 000 years or more. Nevertheless, since it was not until 5, 500 years ago that systematic methods for writing numerals were invented, it was only then that any sort of computation became possible. Which one of the following is an assumption on which the argument relies? ['Grooves and scratches found on bones and stones were all made by people, and none resulted from natural processes.', 'Some kinds of surfaces upon which numeric symbols could have been made in the period before 5, 500 years ago were not used for that purpose.', 'Computation of any sort required a systematic method for writing numerals.', 'Grooves and scratches inscribed on bones and stones do not date back to the time of the earliest people.']