If for a continuous function $f,$

${\int }_{-a}^a f\left(x\right)\mathrm{d}x=K{\int }_0^a \left(f\right(x)+f(-x\left)\right)\mathrm{d}x$ then the value of $K$is

• A

  1

• B

  $1/2$

• C

  2

• D

  $a$