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I'd be happy if someone would answer my questions. Thanks in advance.

Are the following 2 sentences correct? If so, what's the difference?

A: "He jogs every morning. I think it's a good custom."

B: "He jogs every morning. I think that's a good custom."

C: "He jogs every morning. I think this is a good custom."

D: "I'm thinking of taking up jogging. What do you think about it?"

E: "I'm thinking of taking up jogging. What do you think about that?"

F: "I'm thinking of taking up jogging. What do you think about this"
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They are all OK except-- for some reason-- D. In D, it seems to be too vague a pronoun-- does it refer to the thinking or the taking up or the jogging?
This is often used (subconsciusly) by the speaker to draw the topic conceptually closer; it often reveals the speaker's added interest, attaction or positive attitude toward the topic.
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Shaking hands when you meet someone is a custom. Jogging is a habit.
I don't use (and I believe I'm in the majority) the ones with this.
I probably use that most often in these contexts. it seems OK too. I'd probably use A and E.
CJ