We don't know when the accident________.A.has happened B.happened C.happening D.to happen

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Wedon''tknowwhentheaccident________.A.hashappenedB.happenedC.happeningD.tohappenWedon''tknowwhentheacc

We don't know when the accident________.A.has happened B.happened C.happening D.to happen
We don't know when the accident________.A.has happened B.happened C.happening D.to happen

We don't know when the accident________.A.has happened B.happened C.happening D.to happen
B