【单选题】

Each year, millions of people inBangladesh 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, InternationalDevelopmentEnterprises 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 InternationalDevelopmentEnterprises 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 "first-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 3,500 liters of water. InternationalDevelopmentEnterprises 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.
InternationalDevelopmentEnterprises 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, InternationalDevelopmentEnterprises 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.
Which of the following contributes to the low-cost of using rainwaterA．Rainwater is free of chemical treatments.
B．People have been harvesting rainwater for centuries.
C．The water harvesting system is built on a platform.
D．Rainwater can be collected using pipes.