Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: Lewis Hamilton has paid tribute to his team-mate Nico Rosberg after Mercedes celebrated successive team championships on Monday. Both Hamilton, who has one hand on a third Formula One title after his victory in Russia, and Rosberg flew in to the team's headquarters in Brackley and Brixworth to toast Mercedes' constructors' triumph with their 1,100-strong staff. But for Rosberg, the celebrations would have been bittersweet after he retired from Sunday's race at Sochi's Olympic Park with a mechanical issue. The German will now be eliminated from this year's championship battle if Hamilton outscores him by a mere two points at the next race in Austin in a fortnight's time.Mercedes were crowned constructors' champions following Russian GPLewis Hamilton won in Sochi and is closing in on his third drivers' titleThe Briton paid tribute to his team-mate Nico Rosberg during celebrations 
 Question:He is 66 points ahead of Sebastian Vettel and 73 points clear of _ with only four races remaining.

Nico Rosberg