Answer:
All flights were cancelled because of fog.
Explanation:
The sentence provided in the question, " They cancelled all the flights because of [tex]\text{fog}[/tex]." is in the [tex]\text{active voice}[/tex] as the subject [tex]\text{'they' }[/tex] does an action [tex]\text{'cancelled'}[/tex] against object [tex]\text{'flights'.}[/tex]
We have to change this active voice of the sentence into a passive voice sentence.
While changing a sentence from active to passive, the object of the active voice becomes the subject of the passive voice and the verb in the sentence changes to passive form from active form.
Therefore, the passive voice of the sentence would be :
"All flights were cancelled because of fog."
