heteachertoldthemsincelight________(travel)fasterthansound,lightning________(appear)togobeforethunder.

Only when you have obtained sufficient data ________ come to a sound conclusion.

You________gonow.It’sgettinglate.

Wewerebusy________(get)thingsreadyforthetrialproductionwhenhephonedus.

Night________(fall),wehurriedhome.