What does in_reply_to_user_id_str do?

If the represented Tweet is a reply, this field will contain the integer representation of the original Tweet's ID.

If the represented Tweet is a reply, this field will contain the string representation of the original Tweet's author ID

If the represented Tweet is a reply, this field will contain the string representation of the original Tweet's ID.

Details the Tweet ID of the user's own retweet (if existent) of this Tweet.

Answer:
If the represented Tweet is a reply, this field will contain the string representation of the original Tweet's author ID

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