Explanation:

- Option 1: The bell should be rung every forty minutes

- Uses a passive form.

- Suggests a recommendation or expectation but isn't as direct.

- Option 2: Ring the bell every forty minutes

- Direct and simple command.

- Uses active voice.

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- Option 3: The bell ought to be rung every forty minutes

- Similar to Option 1 but with a moral imperative tone.

- Also passive.

- Option 4: Let the bell keep ringing every forty minutes

- Suggests the bell should continue ringing autonomously.

- Implies ongoing activity, possible continuous action.

Option 2 is straightforward and clear.