Given the task definition and input, reply with output. In this task, you're given a question, along with three passages, 1, 2, and 3. Your job is to determine which passage can be used to answer the question by searching for further information using terms from the passage. Indicate your choice as 1, 2, or 3.

Question: What is the other extracellular loop linked by? Passage 1:A return to the Formula Three Euroseries beckoned for van Dam, with a 2009 campaign for Kolles & Heinz Union, the new team set up by Colin Kolles and Werner Heinz. However, the partnership was not to last, as after the rounds at Lausitz, van Dam parted company with the team. In four races, his best finish was eighteenth during the season-opening race at Hockenheim. Van Dam drove in the 24-hour endurance races at the Nurburgring and at Spa, before agreeing to drive the car of PSV Eindhoven in the Superleague Formula series. He replaced Dominick Muermans in the car, with the team lying eighteenth in the overall standings. However, he returned to the Euroseries, for the Barcelona rounds, rejoining his former team SG Formula.
Passage 2:Chemokine receptors are G protein-coupled receptors containing 7 transmembrane domains that are found predominantly on the surface of leukocytes, making it one of the rhodopsin-like receptors. Approximately 19 different chemokine receptors have been characterized to date, which share many common structural features; they are composed of about 350 amino acids that are divided into a short and acidic N-terminal end, seven helical transmembrane domains with three intracellular and three extracellular hydrophilic loops, and an intracellular C-terminus containing serine and threonine residues that act as phosphorylation sites during receptor regulation. The first two extracellular loops of chemokine receptors are linked together by disulfide bonding between two conserved cysteine residues. The N-terminal end of a chemokine receptor binds to chemokine(s) and is important for ligand specificity. G-proteins couple to the C-terminal end, which is important for receptor signaling following ligand binding. Although chemokine receptors share high amino acid identity in their primary sequences, they typically bind a limited number of ligands. Chemokine receptors are redundant in their function as more than one chemokine is able to bind to a single receptor.
Passage 3:Golubic completed his primary education in Stolac, before relocating to Sarajevo to attend high school. In 1908, he moved to Belgrade for post-secondary studies, studying law at the University of Belgrade. Some of Golubic's classmates and contemporaries later recounted that Golubic was recruited by the Russian secret police, the Okhrana, in his youth. The historian Vladimir Dedijer later consulted the records of the Hoover Institution in an attempt to verify this claim, to no avail. Golubic did join Young Bosnia (), a multi-ethnic youth organization agitating for the separation of Bosnia and Herzegovina from Austria-Hungary. The organization's membership was around 70 percent Serb, 20 percent Bosnian Muslim and 10 percent Croat. Following the outbreak of the Balkan Wars in November 1912, Golubic joined the volunteer Chetnik detachment of Major Vojislav Tankosic. As part of their training, Tankosic ordered that Golubic and the other volunteers jump into the Sava from a railway bridge, "just to see whether you are going to fulfill all my orders."
2