Answer:
<u><em>River valley civilizations</em></u>
Explanation:
They were centred around various river valleys as their main source of livelihood and the flourished only due tiote river valley on which they were based. Like the harappan in Indus Valley, The yellow river (Huang He), the Mesopotamia around Tigris and Euphrates and the egyptian Nile.
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<span>The approximate lifetime of a star like our Sun is in the range of 10.3 billion years. At a current age of 4.6 billion years, this will leave (10.3 - 4.6) or approximately 5.7 billion years before the supply of hydrogen fuel (and thereby, helium) will run out.</span>
