You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Mexico_City

Context:
The city's first bus rapid transit line, the Metrobús, began operation in June 2005, along Avenida Insurgentes. Line 2 opened in December 2008, serving Eje 4 Sur, line 3 opened in February 2011, serving Eje 1 Poniente, and line 4 opened in April 2012 connecting the airport with San Lázaro and Buenavista Station at Insurgentes. As the microbuses were removed from its route, it was hoped that the Metrobús could reduce pollution and decrease transit time for passengers. In June 2013, Mexico City's mayor announced two more lines to come: Line 5 serving Eje 3 Oriente and Line 6 serving Eje 5 Norte. As of June 2013, 367 Metrobús buses transported 850,000 passengers daily.

Question:
April 2012 was the year which Metrobus line opened?

line 4