A. 75 and 80
B. 80 and 85
C. 70 and 75
D. 75 and 75
E. 70 and 80

Answer: D

Explanation:
The standard deviation of a set measures the deviation from the mean. Low standard deviation indicates that the points are very close to the mean whereas high standard deviation indicates that the points are spread far apart from the mean.
When we add numbers that are far from the mean, we are stretching the set and hence, increasing the SD. When we add numbers, which are close to the mean, we are shrinking the set and hence, decreasing the SD.
Therefore, adding two numbers which are closest to the mean will shrink the set the most, thus decreasing SD by the greatest amount.
Possibility is 75 and 75.

Hence, the answer is D.

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