1,IhavefinishedalargepartofthenovelwrittenbyDickens,therestofwhich(is)verydifficult.2,Itwas(he)whodidit,butitis(her)thatwearethinkingof.3,Three-fourthsofthesurfaceoftheearth(is)coveredwithwater.4,Thisistheth

　　1,IhavefinishedalargepartofthenovelwrittenbyDickens,therestofwhich(is)verydifficult.

　　2,Itwas(he)whodidit,butitis(her)thatwearethinkingof.

　　3,Three-fourthsofthesurfaceoftheearth(is)coveredwithwater.

　　4,Thisisthethirdtimewhisweekhehashadtostudylate,(isn’tit)?

　　5,Sheisinapoor(state)ofhealth,whichworrieshermothermuch.

　　请解释一下为什么要这样填扩号里的词~