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Help the frustrated theater director explain the disastrous opening night to the press by selecting ALL the grammatically correct sentences that apply.
The correct answers are It was because the lead actor forgot his lines that the audience started laughing. and It was the collapsing scenery, rather than the bad acting, that ultimately ruined the show.
It-clefts use that to connect the emphasized focus to the rest of the sentence.
Even when emphasizing a reason ("because..."), you must use that, not why. Similarly, what is never used as the relative pronoun in a standard it-cleft sentence (though it is used in wh-clefts, such as "What ruined the show was the scenery").