Cov $\left( u _ { i t } , u _ { i s } \mid X _ { i t } , X _ { i s } \right) = 0 \text { for } t \neq s$

cov $\left( u _ { i t } , u _ { i s } \mid X _ { i t } , X _ { i s } \right) = 0 \text { for } t \neq s$
means that

A) there is no perfect multicollinearity in the errors.
B) division of errors by regressors in different time periods is always zero.
C) there is no correlation over time in the residuals.
D) conditional on the regressors, the errors are uncorrelated over time.