Read the passage and answer the questions that follow.

Earth Hour is an annual global campaign that encourages people and businesses around the world to switch off electricity at the same time for one hour at the end of March. Organised by the World Wide Fund for Nature (WWF), Earth Hour started in Australia in 2007 when 2.2 million people in the city of Sydney turned off all non-essential lights for 60 minutes. Since then it has grown to a massive global event. In 2013, more than 7,000 cities and towns worldwide participated in this campaign. Every year landmarks and well-known buildings around the world such as the Sydney Opera House and Harbour Bridge, the Petronas towers in Malaysia, the UK Parliament, Buckingham Palace and the Empire State Building take part and “go dark” for Earth Hour. Therefore, Earth Hour has been a symbolic event to make people think about the problems of climate change and show their commitment to protect the planet.

                                                                                         (Adapted from “Earth Hour” by British Council)

The word “it” in the third sentence refers to __________.