If f(x) is an even function and a is a positive real number, then $$\int_{ - {\text{a}}}^{\text{a}} {{\text{f}}\left( {\text{x}} \right){\text{dx}}} $$ equals
A. 0
B. a
C. 2a
D. $$2\int_0^{\text{a}} {{\text{f}}\left( {\text{x}} \right){\text{dx}}} $$
Answer: Option D

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