⑴as soon as they realized that they had both failed, their enthusiasm faded.
= on realizing that they had both failed, their enthusiasm faded.
= no sooner had they realized that they had both failed than their enthusiasm faded
= hardly(scarcely) had they realized that they had both failed before(when) their enthusiasm faded
같은 문장으로 바꾸어야 하는 문제예요^^