Given the task definition and input, reply with output. This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

In the early 21st century, the population of Paris began to increase slowly again, as more young people moved into the city. It reached 2.25 million in 2011. In March 2001, Bertrand Delanoë became the first socialist mayor of Paris. In 2007, in an effort to reduce car traffic in the city, he introduced the Vélib', a system which rents bicycles for the use of local residents and visitors. Bertrand Delanoë also transformed a section of the highway along the left bank of the Seine into an urban promenade and park, the Promenade des Berges de la Seine, which he inaugurated in June 2013.
What was the population of Paris in 2011?