Articles: a(n), the or no article

Read the explanation and do these exercises:

• http://esl.fis.edu/grammar/rules/article.htm
• http://www.eslmania.com/students/grammar/Grammar_Quizzes/the_a_an.htm
• http://www.better-english.com/grammar/the.htm
• http://www.voanews.com/MediaAssets2/classroom/programs/grammar_bytes/gb_articles/deploy/activity/player.html