Explanation:

- A and B are partners in a partnership firm without any explicit agreement, so default rules apply.

- Partnerships generally distribute profits equally among partners unless otherwise agreed.

- There's no default rule that allows a partner to take an additional commission for devoting more time without an agreement.

- Option 1: 6% of profit - Not applicable without agreement.

- Option 2: 4% of profit - Not applicable without agreement.

- Option 3: 5% of profit - Also not applicable without agreement.

- Option 4: None of the above - Correct, no automatic commission without agreement.

- Option 4 is the correct answer.

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