Highest boiling isomer of C4H11N amines

The same question, asked 2 times: 2 May 2023 S1, 14 May 2024 S1. Every wording below, oldest first.

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Question 1
2 May 2023 S1
68 $Which of the following isomers has highest boiling point?$
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Question 2
14 May 2024 S1
61 $Which isomer among the following has highest boiling point?$
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