△In1850,about a third of U.S.A.( )covered by forests.A.were

△In1850,about a third of U.S.A.( )covered by forests.A.were B.has been C./ D.was △Half of the
△In1850,about a third of U.S.A.( )covered by forests.
A.were B.has been C./ D.was
△Half of the class haven't done all of the homework because some ( )really difficult.
A is B are
请各位亲回答时,说出理由.
为什么third of U.S.A.看作整体,翻译成什么?