You will be given a definition of a task first, then some input of the task.
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: Where was McMahon's post-doctoral mentor from? Passage 1:Ain't She Sweet was an American album featuring four tracks recorded in Hamburg in 1961 by The Beatles featuring Tony Sheridan (except for the title song with vocal by John Lennon) and cover versions of Beatles and British Invasion-era songs recorded by the Swallows. As Atlantic Records only had rights to four Sheridan/Beatle recordings recorded by Polydor Records, they filled the rest of the album with Beatle and British Invasion cover songs. When this material was released by Atco Records, there were mono (catalogue number 33-169) and stereo (SD 33-169) editions. Atco also added additional drum overdubs to the four Sheridan cuts on top of the original drum tracks. American drummer Bernard Purdie claims to have performed overdubs of unspecified Beatles records, which would most likely have been for Ain't She Sweet, but this has never been officially confirmed.
 Passage 2:McMahon contributed to the understanding of retinal neurophysiology alongside his post-doctoral mentor, John E. Dowling. His early research focused on ion channels that mediate transmission at electrical and glutamatergic synapses and the modulatory effects of dopamine and nitric oxide on retinal synapse networks. Through studies with zebrafish he discovered that the neurotransmitter dopamine decreases the electrical coupling within horizontal cells. Further research showed that it was the increase of cAMP within the cell resulting from dopamine binding to AMPA receptor that led to this decrease in coupling. McMahon and his colleagues also demonstrated that exogenous nitric oxide and zinc can modulate AMPA receptor mediated synaptic transmission at gap junctions in hybrid bass retinal neurons.
 Passage 3:The river starts at Ralph Bice Lake (formerly Butt Lake) in northern Algonquin Provincial Park in the geographic township of Butt in the Unorganized South Part of Nipissing District. It flows south to Daisy Lake then east to Big Trout Lake. The river heads north out the lake over Big Trout Lake Dam, takes in the left tributary Tim River, flows over the Portal Rapids, Cedar Rapids, Snowshoe Rapids, Catfish Rapids, and Stacks Rapids to reach Cedar Lake, the location of the community of Brent, where it takes in the left tributary Nipissing River. The river exits the lake over a dam, heads through the Devil's Chute, reaches Radiant Lake, where it takes in the left tributary North River and right tributary Little Madawaska River, and passes through the Squirrel Rapids, Big Sawyer Rapids, Battery Rapids Cascade Rapids and White Horse Rapids, and takes in the right tributary Crow River. The river then continues through a series of rapids including the Devil's Cellar Rapids, passes the Algonquin Radio Observatory, and reaches Lake Travers. The Petawawa River enters a canyon and passes through numerous rapids including the Big Thompson Rapids, Little Thompson Rapids, Grillade Rapids, Crooked Chute, Rollway Rapids, The Natch, Schooner Rapids, Five Mile Rapids to arrive at Whitson Lake adjacent to the Petawawa Hills. It leaves Algonquin Provincial Park and enters the municipality of Laurentian Hills in Renfrew County, continues southeast past CFB Petawawa, passes through the Crooked Rapids, Race Horse Rapids, White Horse Rapids and Halfmile Rapids, and reaches Lac du Bois Dur, where it takes in the right tributary Barron River. The river enters the town of Petawawa, heads under Ontario Highway 17, through the Big Eddy Rapids, under the Canadian Pacific Railway main line, and empties into Black Bay on the Ottawa River.

Output:
2