Which of the following statement about the accuracy estimation of the least squares method is incorrect ( ).

A、The constraint condition is that the sum of squares of residuals is 0;

B、The accuracy of the estimator is related to the accuracy of the measured data;

C、 The estimator is obtained by the direct measurement and the functional relationship between the direct measurement and the quantity to be obtained;

D、The estimation results obtained by the least square method are unbiased