Passive : advanced structures

Prosím prvé cvičenie aj druhé. Ďakujem

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Dobry vecer, dokedy to potrebujete?
druhé cvičenie takto :)
1-Two thousand years ago, the earth was known to be round
2-The dinosaurs are believed to have died out because of a huge meteor impact.
3-There are known to have been jellyfish in the sea for more than 500 million years.
4-Today, the earth is known to be 4.5 billion years old.
--ak by si chcel aj bližšie vysvetlenia prečo tak napíš ;)