Explanation:

- A can finish the work alone in 18 days.

- A works alone for 9 days. In these 9 days, A completes 9/18 = 1/2 of the work.

- Half of the work remains after A's 9 days of effort.

- A and B work together for 4 more days to finish the remaining half.

- In 4 days, A and B complete the remaining 1/2 work.

- So, in 4 days, A and B together can complete what A alone does in 18 days.

- To calculate how long A and B would take to complete the whole work together, we know:

- 4 days for 1/2 work means both can finish the whole work in 4 * 2 = 8 days.

- Option 4: 8 days is the correct choice.

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