8. If $P ( 1,2 ) , Q ( 4,6 ) , R ( 5,7 )$ and $S ( a , b )$ are the vertices of a parallelogram $P Q R S$, then :\\
(A) $\mathrm { a } = 2 , \mathrm {~b} = 4$\\
(B) $\mathrm { a } = 3 , \mathrm {~b} = 4$\\
(C) $\mathrm { a } = 2 , \mathrm {~b} = 3$\\
(D) $\mathrm { a } = 3 , \mathrm {~b} = 5$\\
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