change the verbs in brackets into the passive and complete the sentences.(1) the sahara was not always a desert millions of years ago_________(cover) in grasslands and forests. now, only a tenth of the sahara desert____________(cover)with wind-blown sand. the rest is plains and mountains.(2) many stars in the universe are bigger than our sun. the biggest star__________(estimate) to be over 700 times as big as the sun. so, these stars________(know) as blue giants.