Read the passage below carefully, and then choose the best answer:
Make Home a Little Nicer
Some people think that if they don’t have a yard, they can’t keep a garden. Fortunately, that’s not true. For people who live in apartment buildings, there is another option: rooftop gardens. A rooftop garden is just a garden that you keep on your roof. Rooftop gardens are very popular in cities. Most building roofs go to waste – people don’t use that space for anything. Putting a garden on your roof is a nice way to turn an empty space into something that is pretty and relaxing. Rooftop gardens are not a new idea. About 2,600 years ago, people in Ancient Mesopotamia planted rooftop gardens. There were similar gardens in Ancient Rome and Ancient Egypt. These gardens had a very important purpose. Many ancient cities had walls. The enemy army usually tried to destroy the city’s food, so the people in the city kept their gardens on top of their walls. That way, they could continue to grow food. Today’s rooftop gardens don’t keep us alive, but they still have nice benefits. Plants absorb heat, so if there are a lot of them on a rooftop, they make the whole building cooler. As a result, people in the building use the air conditioning less which means they use less electricity. One study said that if every roof in Tokyo had a garden, the city would save more than one million dollars a day on electricity. Rooftop gardening is harder than regular gardening. Here are some tips. First of all, be safe. Build a fence around your garden. You don’t want the wind to blow plants off your roof! Also, remember that rooftop gardens get more sun. If you have a plant that shouldn’t get too much sun, remember to put it in the shade sometimes. There are many other things that you need to know before you start a rooftop garden. Do some research and then give it a try. You’ll make your little part of the city much nicer.
4. Why should you build a fence around a rooftop garden?

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