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Scientists have to work hard to keep _____ with modern discoveries and developments.   

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used to work    am used to work    am used work    am used to working  
work   working   to work   works  
work  working  to work  works  
company    track    touch    pace  
have gone    will have gone    have been going    had been going  
should    shall    must    have to  
should study    may have studied    should have studied    must have studied  
should    shall    must    have to  
have been going    have gone    was going    will have gone  
promotion  advance   accession  progress  
should    shall    must    have to  
have gone    have been going   was going    will have gone  
have been going    have gone    was going    will have gone  
have gone    have been going    was going    will have gone   
have been going    have gone    was going    will have gone  
have gone    have been going    was going   will have gone  
must have worked    should have worked    may have worked    should work  
have been going  was going  have gone  will have gone  
but    so    although    because  

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