仁爱初一英语小问题书上第81页有一句话：SOthat'stwobagsofsalt,twokilosofeggsandsixbottlesofmilk.为什么这里用that's而不用thoseare?

　　仁爱初一英语小问题

　　书上第81页有一句话：SOthat'stwobagsofsalt,twokilosofeggsandsixbottlesofmilk.为什么这里用that's而不用thoseare?