杨鹏长难句里面的一个句子thisis,thedepletionofavailablenewland...
ThehistorianFrederickJ.Turnerwroteinthe1890'sthattheagrariandiscontentthathadbeendevelopingsteadilyintheUnitedStatessinceabout1870hadbeenprecipitatedbytheclosingoftheinternalfrontier——thatis,thedepletionofavailablenewland neededforfurtherexpansionoftheAmericanfarmingsystem.
thisis到后面的部分是什么成分?修饰哪里?