If $x_i$'s are the midpoints of the class intervals of grouped data, $f_i$'s are the corresponding frequencies and I is the mean, then $\sum (f_i x_i - \bar {x} )$ is equals to ______

Question

$(A) \; 0 \\(B)\;-1 \\(C)\; 1 \\(D)\; 2$