Let f and g be continuous fuctions on [0, a] such that `f(x)=f(a-x)" and "g(x)+g(a-x)=4 " then " int_(0)^(a)f(x)g(x)dx` is equal to
A. `4underset0oversetaintf(x)dx`
B. `2underset0oversetaintf(x)dx`
C. `-3underset0oversetaintf(x)dx`
D. `underset0oversetaintf(x)dx`
