\[\int_{0}^{a}{\frac{x\,dx}{\sqrt{{{a}^{2}}+{{x}^{2}}}}}=\]

• A. \[a\,(\sqrt{2}-1)\]
• B. \[a\,(1-\sqrt{2})\]
• C. \[a\,(1+\sqrt{2})\]
• D. \[2a\sqrt{3}\]