Explanation:

- The total amount of work is constant. If 25 people can finish it in 140 days, the total work is the same regardless of the time frame.

- The work done by one person in one day is a unit you can base calculations on.

- If the required timeline is halved (from 140 to 70 days), the workforce must double to maintain the same amount of work completed within this shorter period.

- The calculation: $(25\text{ persons}\times 140\text{ days})=(X\text{ persons}\times 70\text{ days})$ results in $X=50$ persons.

- Option 2: 50 ? is the correct answer as it reflects the workforce needed to complete the work in half the time initially allotted.

- The other options do not correctly represent the workforce required for doubling the work speed to meet the shorter deadline.