Re: sentence two - when you use "the class," this is the equivalent of "it," and for conjugation purposes also the equivalent of "he/she" so you must remember to add an 's' to your verb - so not pay, but pays.
Re: sentence one - we would "put emphasis on reducing air pollution."
Re: sentence two - "The training class places great emphasis on writing skills."
Re: sentence three - "The food industry has put great emphasis on marketing to young customers."