$P,Q$ have position vectors $\overrightarrow { a } $ and $\overrightarrow { b }$ relative to the origin $O$ and $X,Y$ divide $\overrightarrow { OQ }$ internally and externally respectively in the ratio $2:1$. Vector $\overrightarrow { XY }=$
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$\displaystyle \dfrac { 3 }{ 2 } \left( \overrightarrow { b } -\overrightarrow { a } \right) $
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$\displaystyle \dfrac { 3 }{ 4 } \left( \overrightarrow { b } -\overrightarrow { a } \right) $
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$\displaystyle \dfrac { 5 }{ 6 } \left( \overrightarrow { b } -\overrightarrow { a } \right) $
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$\displaystyle \dfrac { 4 }{ 3 } \left( \overrightarrow { b } -\overrightarrow { a } \right) $