文（句、節）を指すItとThat,Thisの違い
(4)前に出た句や節、文の内容をさす [ロイヤル英文法p.176]
・I'd like to go on a trip to Europe, but I cannot afford it. (=to go on a trip to Europe)
・Railroad service was suspended. They told me it was because of a landslide. (itは前文の内容)
(3)this, that が先行する文（の一部）の内容をさす場合 [ロイヤル英文法p.197]
・To be or not to be : that is the question.
・They will surely help Tom. That will please his mother.
・"I've caught a cold." "That's too bad."
・We see him taking a walk, but this isn't very often.
It,That,Thisはセンテンス全体を指せるようですが、それぞれの違いはなんですか？
お礼
ありがとうございました。助かりました。