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若 $\lim \limits_ {x \rightarrow 0} \dfrac {x- \sin ax}{ \int _{0}^{x} \dfrac {t^{2}}{ \sqrt {b t^{4}}}dt}=2$, 则$\left(\quad\right)$.
A. $a = 1$,$b = 2$     B. $a = 1$,$b = 4$     C. $a = 1$,$b = 6$     D. $a = 1$,$b = 16$         
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