What's troubling them is____enough experienced workers.为什么不能

What's troubling them is____enough experienced workers.为什么不能 为什么不能说what's your work? what's matter with them? these teams.which( )twenty experienced workers.are working h what (sets) them apart is hope.请问这里的sets为什么要加S? each of them knows what the others are doing.这里的knows为什么加s "cut them up "为什么不能换成“cut up them ” What's the difference between them? What's the relationship between them? what's in it for them 是什么意思 --what's this?--_____is a pen.为什么要填these?为什么不能填it? what's happened to them 's=has?