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We have been excavating layers of ash almost six metres thick, which suggests that they________the f...

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loads  layers  pyramids  piles  
should keep    might have kept    should have kept    might keep  
work    are working    have been working    have been worked  
have been to    have been in    have been    have gone  
Scientists have been trying to break the ice-rich layers of soil on Mars.  Scientists have been surprised by how the soil on Mars behaves.  Scientists have been trying to fend out if there is life supporting material on Mars.  Scientists have been trying to know if water ice will melt.  
We have gone to    Though we have gone to    We have been to    Having been to  
had been; have seen  have been; have seen  had been; had seen  have been; had seen  
loads  layers  pyramids  piles  
had been; have seen    have been; have seen    had been; had seen    have been; had seen  
had been; have seen    have been; have seen    have been; had seen    had been; had seen  
might have kept    should keep   should have kept    have kept  
have arrived , have been   have come , have been    came , have come    arrived , have been  
we have been treated  have we been treated  we have treated  have we treated  
promoted    had been promoted    have promoted    have been promoted  
loads  layers  pyramids  piles  
loads  layers  pyramids  piles  

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