What is \(\smallint \frac{{{\rm{dx}}}}{{\sqrt {{{\rm{x}}^2} + {{\rm{a}}^2}} }}\) equal to?

• A. \(\ln \left| {\frac{{{\rm{x}} + \sqrt {{{\rm{x}}^2 + a^2}}}}{{\rm{a }}}} \right| + {\rm{c}}\)
• B. \(\ln \left| {\frac{{{\rm{x}} - \sqrt {{{\rm{x}}^2} + {{\rm{a}}^2}} }}{{\rm{\alpha }}}} \right| + {\rm{c}}\)
• C. \(\ln \left| {\frac{{{{\rm{x}}^2} + \sqrt {{{\rm{x}}^2} + {{\rm{a}}^2}} }}{{\rm{\alpha }}}} \right| + {\rm{c}}\)
• D. None of the above