Why were the French forced to start the bounty program to kill rats?

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The French were forced to start the bounty program to kill rats-

•The French built a modern city in Hanoi for the French citizens.

•They built the large sewers in that city as a symbol of modernity.

•The refuse from the old city in rainy season drained and flooded the streets of Hanoi.

•It proved to be an ideal and protected breeding ground for rats.

•Rats soon began to enter the well cared for homes of the French through the sewage pipes.

•To stop this rat menace, rat hunt bounty program was started in 1902.

•The French hired Vietnamese workers to caught rats and paid them for each rat they caught.

The bounty was paid to them when a tail was shown as proof that the rat had been killed.