What is this passage mainly about?h

Read the following passage and mark the letter A, B, C, or D to indicate the answer to each of the question. 

The Urban Gardener

      Cities are home to skyscrapers and apartment buildings, and it's rare to find wide, open spaces within them. With limited space for parks and gardens, architects and city planners often find it challenging to incorporate greenery into neighbourhoods. 

      One creative solution is to grow plants on unused areas like walls or rooftops. It's a popular idea, and now rooftop gardens and green walls have been spouting up in cities around the world. There are many benefits to having green spaces to the urban landscape. Adding gardens to rooftops or walls can create a pleasant environment - what was once a grey cement wall can become a colourful, blooming garden. The CaxiaForum art gallery in Madrid, Spain, is a famous example - one of its walls is covered with 15,000 plants from over 250 different species. 

      In other cities, green walls are being used more functionally, to cover up construction sites and empty buildings and to prominently decorate the lobbies of office buildings. Using plants to cover walls and rooftops can also keep cities cooler in the summer. Buildings and roads absorb the sun's heat and hold it, causing a building or neighbourhood to stay warmer longer. Plants, on the other hand, provide an enormous amount of shade. There is evidence that growing a roof or wall garden can lowe Many cities offer tax discounts to businesses with these features. 

      In New York City, public schools plant rooftop gardens that can reduce heating and cooling costs. In addition to saving the school money, teachers and parents love the gardens because of their educational value - it's a fun and healthy way for their kids to investigate the world around them. “For the children, it's exciting when you grow something edible," said Lauren Fontana, principal of a New York public school. 

      These green spaces are also used to grow food. In recent years, rooftop gardens have slowly been included in the "local food movement". This is based on the concept that locally grown food reduces pollution since it does not have to be transported far. Vegetables are being grown in rooftop gardens by schools, churches, neighbourhoods and even restaurants. Chef Rick Bayless serves “Rooftop Salsa” at his restaurant in Chicago, USA, using only ingredients grown in his rooftop garden. Rooftop gardens and green walls may require a bit more effort to grow and maintain. However, hard work always brings rewards, and with green spaces, the rewards are plentiful. 

What is this passage mainly about?

• A. gardens in Madrid, New York, and Chicago
• B. people growing plants on roofs and walls
• C. private gardens in the city's unused spaces
• D. how to grow your own food in the city