"None" is a your clue word... if "none" means zero, then there is still "6 birds" because 0 left.
In a equation it will look like this: 6 - 0 = 6
Based on the options given, the answer should be the standard deviation of the information given which are:SSD=184N=8
V2= 184/8-1 = 26.29SD= square root of 26.29 = 5.13
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6,4 and or 8,7 should be the answer