For an experiment we have the following data set: n = 4, $$\sum_{}^{}X = a, \sum_{}^{}Y = 10, \sum_{}^{}XY = 21,\sum_{}^{}X^{2} = 30, \sum_{}^{}Y^{2} = 30$$ the correlation coefficient is -0.8 then the value of a is:

A

8

B

7

C

10

D

9