An econometrician determines the right set of controlvariables:

A.

by adding all variables that you have collected data for.

B.

so that E(ui Xi, Wi) = E(ui Wi) where Xi is(are) thevariable(s) of interest and Xi is(are) the controlvariable(s).

C.

so that E(ui Xi, Wi) = E(ui Wi) where Xi is(are) thevariable(s) of interest and Wi is(are) the controlvariable(s).

D.

by adding variables until the R2 does not increase any more.