Choose the best answer to transform the sentence from the direct speech to the reported speech.
The professor said: ''Hunger is a problem because of low yieds in farming in many countries''.
The professor said that _______.
hunger was a problem because of low yieds in farming in many countrieshunger were a problem because of low yieds in farming in many countrieshunger had been a problem because of low yieds in farming in many countrieshunger has been a problem because of low yieds in farming in many countries