Why is the green wall in Madrid mentioned in the passage?

Read the following passage and mark the letter A, B, C or D on your answer sheet to choose the word or phrase that best fits each other numbered blanks. 

WHAT IS URBANIZATION?

     Urbanization is a process (23) ________ populations move from rural to urban areas, enabling cities and towns to grow. It can also be termed as a progressive increase in the number of people living in towns and cities. It is highly influenced by the notion that cities and towns have achieved better economic, political, and social mileages compared (24) _______ the rural areas. Accordingly, urbanization is very common in developing and developed worlds as more and more people have the (25) _______ of moving closer to towns and cities to acquire “privileged” social and economic services. These (26) _______ social and economic advantages such as better education, health care, sanitation, housing, business opportunities, and transportation. A large majority of people move to cities and towns because they view rural areas as places with hardship and backward lifestyle. Therefore, as populations move to more developed areas, the immediate outcome is urbanization. This normally contributes to the development of land for use in commercial properties, transportation, and residential buildings. Eventually, these activities (27) _______ several urbanization issues.