B) is correct. A) if you change 'when' to 'that'. I don't know what book you are referring to.
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Gu-Hoon KwonWhich is right, A or B?A is. For B to be correct, it would have to be It was at eleven that he came back home (He came back home at eleven).
Aspara GusA is. For B to be correct, it would have to be It was at eleven that he came back home (He came back home at eleven).May I ask why what wouldn't work here? I know it's incorrect but I don't know why.
Perfect StrangerMay I ask why what wouldn't work here?Because the sentence uses an "it-cleft" construction, in which what doesn’t work. You’d need a "wh-cleft": Eleven is what time he came home at, but a native would be more likely to say Eleven is when he came home.
Aspara GusA is. For B to be correct, it would have to be It was at eleven that he came back home (He came back home at eleven).