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. █████████████ █████ ██ ███ ███ █████ █████ █████ ███ ████ ██████████ ███████ ███ ███████ ████████ ████ █████████ ██ ███ ████ ████ ████ ███ ████ ██ ███████████ ██████ █████████
The argument discusses number symbols, which have been around in some form for at least 20,000 years. Nevertheless, the argument concludes that computation was impossible until 5,500 years ago. Why? Because we didn't have systematic methods for writing number symbols until that time.
The argument relies on the assumption that a systematic method for writing number systems is a necessary condition for computation. Otherwise, there would be no reason that an absence of such a method would prevent us from developing computation before 5,500 years ago.
Which one of the following ██ ██ ██████████ ██ █████ ███ ████████ ███████
Grooves and scratches █████ ██ █████ ███ ██████ ████ ███ ████ ██ ███████ ███ ████ ████████ ████ ███████ ██████████
The argument already tells us that
Some kinds of ████████ ████ █████ ███████ ███████ █████ ████ ████ ████ ██ ███ ██████ ██████ █████ █████ ███ ████ ███ ████ ███ ████ ████████
The argument tells us all we need to know about number symbols: before 5,500 years ago, we only had
Grooves and scratches █████████ ██ █████ ███ ██████ ██ ███ ████ ████ ██ ███ ████ ██ ███ ████████ ███████
The argument only discusses ancient, informal number symbols as context. The substance of the argument is about the development of systematic number symbols allowing the possibility of computation. New details about informal number symbols aren't necessary.
Computation of any ████ ████████ █ ██████████ ██████ ███ ███████ █████████
The argument concludes that computation was impossible before 5,500 years ago, because there was no systematic method for writing numerals. Without (D), this wouldn't make any sense—if systematic numerals weren't necessary, computation could have been developed earlier.
Systematic methods for ███████ ████████ ████ ████████ ████ ███████ ███ ████ ███ ███████████ ██████
It doesn't matter to the argument why systematic methods for writing numerals came about, only their effect on computation. Whether these methods were invented to enable computation or just by coincidence, the argument remains plausible.