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If f(x)=f(a+x) then $\int_{a}^{ka} f(x)dx$ equals:
Difficulty: Easy
A: 
$(k+1)\int_{b}^{a}f(x)dx$
B: 
$k\int_{0}^{a}f(x)dx$
C: 
$(k-1)\int_{0}^{a}f(x)dx$
D: 
0