Let $f : [ - 1,2 ] \rightarrow [ 0 , \infty )$ be a continuous function such that $f ( x ) = f ( 1 - x )$ for all $x \in [ - 1,2 ]$. Let $R _ { 1 } = \int _ { - 1 } ^ { 2 } x f ( x ) d x$, and $R _ { 2 }$ be the area of the region bounded by $y = f ( x ) , x = - 1 , x = 2$, and the $x$-axis. Then
(A) $R _ { 1 } = 2 R _ { 2 }$
(B) $R _ { 1 } = 3 R _ { 2 }$
(C) $2 R _ { 1 } = R _ { 2 }$
(D) $3 R _ { 1 } = R _ { 2 }$