The correct option is D (1) flew, (2) flu, (3) flue
In the first sentence, ''The bird flew over the trees'' is correct because flew implies to be carriedthroughtheair by thewind. Flue is a passage or ductforsmoke in a chimney. Flu is a specificvariety of influenza.
In the second sentence, ''She was down with the flu yesterday'' is correct because flu is a specificvariety of influenza. Other options are incorrect.
In the third sentence, " In all cases, there should be a flue to let the gases of the flames escape", flue is correct because flue is a passage or ductforsmoke in a chimney. Other options are incorrect.