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A little extra background on standard deviations above and below the mean

If, for example, a set has a standard deviation of 4, then:
1 standard deviation = (1)(4) = 4
2 standard deviations = (2)(4) = 8
3 standard deviations = (3)(4) = 12
1.5 standard deviations = (1.5)(4) = 6
0.25 standard deviations = (0.25)(4) = 1
etc
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So, if the mean of the sample is 23, and the standard deviation is 3.5, then:
2 standard deviations ABOVE the mean = 23 + 2(3.5) = 30
2 standard deviations BELOW the mean = 23 - 2(3.5) = 16

So, values that are LESS THAN 2 standard deviation from the mean, will be all values between (but not including) 16 and 30

Answer: D, E, F