Read the passage. Earthships

­­­­­What is an earthship?

An earthshi­­­p is a home designed to make use of recycled materials and increase energy conservation. Ideally, by living in an earthship, a person can have a home that is “off the grid.” People who live in earthships do not depend on outside sources for electricity, food, or water. Building an earthship may be an attractive choice for those who want to use fewer of our planet’s non-renewable resources.

The Foundation

First, the foundation is built by firmly packing dirt inside recycled tires. Then, the tires are placed in a pyramid-like stack going as high as needed. Next, cement is spread and smoothed between the tires to create a solid wall. When this is finished, the walls are sealed with a protective coating and painted. Some interior walls are built using recycled cans and cement, also sealed with a protective coating.

Solar Energy

Solar panels give enough stored energy in batteries to provide electricity for appliances, lighting, and electronics. However, solar-powered batteries hold about one-third the charge of energy used in a regular household wired for electricity. This means that people either buy more batteries or intend to use less electricity than a conventional household. Ideally, earthships are built in places where there is an abundance of sunshine year-round.

Floor-to-ceiling windows on an earthship’s south wall also allow plenty of sunlight. It is important that no trees block the light on this side of the house. The sunlight shines directly onto a brick floor, which then absorbs the heat. This provides enough warmth to maintain a comfortable temperature in the house for the rest of the day. This method of heating the home is called passive solar. Most earthships also have either a wood stove or a heater that uses propane, a type of natural gas. These heat sources are useful on cloudy days. During the summer, the combination of the cool temperature of the earth beneath the floor plus the thick walls can keep the house comfortable without the use of air-conditioning.

Recycled Water

Gutters on the roof collect rainwater that then trickles down into large storage barrels. The water is used for taking showers, doing dishes, and flushing the toilet. It is also filtered for drinking. People sometimes build a greenhouse to grow their own food. Water that has been used for dishwashing or showers can be saved if the soaps are chemical-free. This recycled “gray” water can be used yet again to water the plants in the greenhouse.

Potential Problems

Earthships have been around since the 1970s. Now, ­­long-term studies have revealed some problems. For one, without sufficient sunlight during the winter, large amounts of natural gas known as propane and/or wood are used to heat the home. Propane use can be costly, and unless one has planned far ahead, a wood supply can be quickly depleted.

Another problem is that tires used in the foundation walls can, after a long period of time, begin to crack, releasing a toxic gas that has built up over time in the walls. The use of cement, which is a porous material with many small holes and spaces, allows the gases to leak into the air. The type of gas emitted is not detectable by smell but can make people sick. To address this problem, the walls of the home needs to be resealed every year.

Cost can also be a significant challenge. Some earthship building companies claim that it is far cheaper to build an earthship because only recycled materials are used. Also, a contractor’s license or training is not needed to build one. This, though, is true if it is built 100% by the owner, which could take years to complete. Additionally, the cement, plumbing, and electrical components, as well as the cost of installing solar panels, can be expensive. Just as with the construction of a conventional home, proper permits are often needed to build an earthship.

There are clear advantages and disadvantages to these unique homes. As technology improves and new solutions are discovered, earthships may continue to be a wise way to live sustainably using minimal resources.

Based on the passage "Earthships," what does the term gray water mean?

A). reusable wastewater

B). pure spring water

C).gutters that collect rainwater

D).filtered drinking water