Goal takes a look at the key factors in determining which two of Arsenal, Marseille, Dortmund and Napoli will be taking their place in the last 16

Every year, the Champions League throws up a group of obscene difficulty, with four of Europe's finest teams going head to head for two places in the latter stages of the competition.
This year, Arsenal, Olympique de Marseille, Borussia Dortmund and Napoli will do battle for a last-16 berth. Each side possess some of the best players on the planet, with Mesut Ozil of the Gunners facing the likes of Gonzalo Higuain, Ilkay Gundogan and Dimitri Payet.
Here, Goal profiles the four unlucky teams and examines their respective chances of making it past the first phase of the competition.
By James Goldman
Though Arsenal have been handed their toughest group stage assignment since their second season in the competition, they should meet the challenge ahead without any form of inferiority complex. 
Back in 1999, Arsenal, and to some extent Arsene Wenger, were European novices playing their home matches at Wembley. Pitted against a Rivaldo-inspired Barcelona and a Gabriel Batsituta-led Fiorentina they were comprehensively outclassed. 
Arsenal may not be the dominant pacesetters they once were at home, but in Europe they are now, if not a force, an accomplished and highly respected club - recent history stands them in good stead. 
At this stage of the competition two seasons ago Arsenal took eight points from four fixtures against Marseille and Borussia Dortmund, while Arsene Wenger’s head-to-head record against Italian opposition and Rafa Benitez provides a significant source of encouragement. 
If they are to progress, however, maintaining a relatively clean bill of health is essential. Already their forward options have been severely hampered by injuries to Lukas Podolski and Alex Oxlade-Chamberlain, while Olivier Giroud is nursing a knee injury. 
An abundance of midfield options, bolstered by the arrival of assist king Mesut Ozil, and an improving, if far from faultless, defence, will count for little if Giroud and even Theo Walcott were to miss decisive fixtures.