Problem: The Centro Nacional de las Artes (National Center for the Arts has several venues for music, theatre, dance. UNAM's main campus, also in the southern part of the city, is home to the Centro Cultural Universitario (the University Culture Center) (CCU). The CCU also houses the National Library, the interactive Universum, Museo de las Ciencias, the Sala Nezahualcóyotl concert hall, several theatres and cinemas, and the new University Museum of Contemporary Art (MUAC). A branch of the National University's CCU cultural center was inaugurated in 2007 in the facilities of the former Ministry of Foreign Affairs, known as Tlatelolco, in north-central Mexico City.
Where is Tlatelolco located?
The answer is the following: north-central Mexico City

Problem: Additionally, cable-ready TV sets can display HD content without using an external box. They have a QAM tuner built-in and/or a card slot for inserting a CableCARD.
What features allow the cable-ready TVs to display HD content without an external box?
The answer is the following: a QAM tuner built-in and/or a card slot for inserting a CableCARD

Problem: Railways must operate at variable speeds. Until the mid 1980s this was only practical with the brush-type DC motor, although such DC can be supplied from an AC catenary via on-board electric power conversion. Since such conversion was not well developed in the late 19th century and early 20th century, most early electrified railways used DC and many still do, particularly rapid transit (subways) and trams. Speed was controlled by connecting the traction motors in various series-parallel combinations, by varying the traction motors' fields, and by inserting and removing starting resistances to limit motor current.
Why is DC motor being used more than AC type?
The answer is the following: conversion was not well developed

Problem: Another aspect of anti-aircraft defence was the use of barrage balloons to act as physical obstacle initially to bomber aircraft over cities and later for ground attack aircraft over the Normandy invasion fleets. The balloon, a simple blimp tethered to the ground, worked in two ways. Firstly, it and the steel cable were a danger to any aircraft that tried to fly among them. Secondly, to avoid the balloons, bombers had to fly at a higher altitude, which was more favorable for the guns. Barrage balloons were limited in application, and had minimal success at bringing down aircraft, being largely immobile and passive defences.
What did pilots have to do to avoid the barrage balloons?
The answer is the following:
fly at a higher altitude