【案例分析题】

Each year，millions of people in Bangladesh drink ground water that has been polluted by naturally high levels of arsenic poison.Finding safe drinking water in that country can be a problem.However，International Development Enterprises has a low-cost answer.This non-governmental organization has developed technology to harvest rainwater.
People around the world have been harvesting rainwater for centuries.It is a safe，dependable source of drinking water.Unlike ground water，rainwater contains no minerals or salts and is free of chemical treatments.Best of all，it is free.
The rainwater harvesting system created by International Development Enterprises uses pipes to collect water from the tops of buildings.The pipes stretch from the tops of buildings to a two-meter tall storage tank made of metal.At the top of the tank is a so-called ’lust-flush’device made of wire screen.This barrier prevents dirt and leaves in the water from falling inside the tank.
A fitted cover sits over the ’first-flush’device.It protects the water inside the tank from evaporating.The cover also prevents mosquito insects from laying eggs in the water.
Inside the tank is a low coat plastic bag that collects the water.The bag sits inside another plastic bag similar to those used to hold grains.The two bags are supported inside the metal tank.All total，the water storage system can hold up to three-thousand-five-hundred liters of water.International Development Enterprises says the inner bags may need to be replaced every two to three years.However，if the bags are not damaged by sunlight，they could last even longer.
International Development Enterprises says the water harvesting system should be built on a raised structure to prevent insects from eating into it at the bottom.The total cost to build this rainwater harvesting system is about forty dollars.However，International Development Enterprises expects the price to drop over time.The group says one tank can provide a family of five with enough rainwater to survive a five-month dry season.

People in Bangladesh can use（）as a safe source of drinking water.