Choose the correct option.

Choose the correct option.
Once 23-year-old Mark , 1) ... a social network while he 2) ... at the university. People 3) ... a university website with students’ profiles, so Mark 4) ... to do something about it. 1,200 students 5) ... within 24 hours! He then 6) ... the site to include other universities. Mark 7) ... some difficulties, though. Three seniors 8) ... that while Mark 9) ... on a similar project with them, he 10) ... their ideas to create one of the most popular social networks in the world. This 11) ... Mark, though, and his network soon 12) ... the biggest social networking site in the world with 500 million users to date.