The population of a city increases at a rate proportional to the population at that time. If the population of the city increases from $20$ lakhs to $40$ lakhs in $30$ years,then after another $15$ years,the population is:
- A$10 \sqrt{2}$ lakhs
- B$40 \sqrt{2}$ lakhs
- C$30 \sqrt{2}$ lakhs
- DNone of these
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