Answer:
Variance of difference between pre-exam and post-exam grades = 10.8
Step-by-step explanation:
Correlation (X,Y) = Covariance (X,Y) / √ { Variance (X) . Variance (Y) }
Given : Correlation (Pre & post exam grade) = 0.9 ; Variance (Pre exam grade) = 0.90 ; Variance (Post exam grade) = 16
So, 0.9 = Covariance (X,Y) / √ { 16 x 9 }
0.9 = Covariance (X,Y) / 12
Covariance (X,Y) = 12 x 0.9 = 10.8
the answer is D because it is the only answer that was clearly defined in the question.
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The answer is b! If it’s not then it’s A! Hope I was able to help ! Mark me brainly if you can.