When ................... is not known. A. was the wheel ...

[ Adverb and Noun Clauses ]

When ………………. is not known.

A. was the wheel invented

B. the invention of the wheel

C. inventing the wheel

D. the wheel was invented

Select your answer:

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Modal Verbs and Conjuctions In/On/At + Time Modifiers Grammar, Literature, and Vocabulary Present Perfect Time Expressions Possessive Adjectives PRESENT SIMPLE - CHILDREN Clauses of Reason

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