Let f, be a continuous function in [0,1], then \lim _{n \rightarrow \infty} \sum_{j=0}^{n} \frac{1}{n} f\left(\frac{j}{n}\right) is
(A) \frac{1}{2} \int_{0}^{\frac{1}{2}} f(x) d x
(B) \int_{\frac{1}{2}}^{1} f(x) d x
(C) \int_{0}^{1} f(x) d x
(D) \int_{0}^{\frac{1}{2}} f(x) d x