Itisusefultoscience_______theearthasanobjectinspace.
Whenworkersareorganizedintradeunions,employersfindithardtolaythem________.
Theboyranoutoftheclassroomwithouttheteacher's_______.
Othersarguethatsincetheearthhasenduredalongstringoficeagesinthelasttwomilliontothreemillionyears,anywarmingis________tobetemporary.