 Complete the following phrase:  From Aztec times, the Centro Historico used to be where the wealthy and elite lived. However, in the early 20th century, these classes began to move to areas west and southwest of the Centro, to neighbourhoods such as Colonia Juárez, Colonia Cuauhtémoc, Colonia Roma and Colonia Condesa. The Centro remained the commercial, political and intellectual center through the mid 20th century although it was around this time that UNAM moved most of its facilities to the new Ciudad Universitaria. The reason for the decline of the city center was partly man-made and partly natural. In the 1940s, the city government froze rents so that until 1998 when the government repealed the law, tets were still paying 1950s-level rents. With no ficial incentive to keep up their properties, landlords let their buildings disintegrate. The 