Please answer the following question: Extract the answer to the following question from the movie plot. If the question isn't answerable, please output "Can't answer". Question: What prototype is under heavy guard? Title: Firefox Movie plot: MIG-31 Firefox A joint Anglo-American plot is devised to steal a highly advanced Soviet fighter aircraft (MiG-31, NATO code name "Firefox") which is capable of Mach 6, is invisible to radar, and carries weapons controlled by thought. Former United States Air Force Major Mitchell Gant (Clint Eastwood), a Vietnam veteran and former POW, infiltrates the Soviet Union, aided by his ability to speak Russian (due to his having had a Russian mother) and a network of Jewish dissidents and sympathizers, three of whom are key scientists working on the fighter itself. His goal is to steal the Firefox and fly it back to friendly territory for analysis. However, the KGB has got wind of the operation and is already hot on Gant's tail. It is only through the sympathizers that Gant remains one step ahead of the KGB and reaches the air base at Bilyarsk, where the Firefox prototype is under heavy guard. The dissident scientists working on the Firefox help Gant infiltrate the base. Dr. Pyotr Baranovich (Nigel Hawthorne), one of the scientists, informs Gant that there is a second prototype in the hangar that must be destroyed. The diversion will allow Gant to enter the hangar and escape with the first Firefox. Gant knocks out Lt. Colonel Yuri Voskov (Kai Wulff), a Soviet pilot assigned to take the first prototype on its maiden flight during a visit from the Soviet First Secretary. The scientists attempt the destruction of the second prototype to give Gant time to suit up and start the first fighter, but the second prototype is not destroyed, and the scientists are killed by the guards. Fortunately for Gant, he escapes the hangar and lifts off just as the First Secretary arrives. Evading the Soviets' attempts to stop him, Gant barely reaches the Arctic ice pack and lands for refueling, making a rendezvous with a US submarine whose crew refuels and rearms the aircraft. However, Gant's last-minute refusal to kill Voskov has consequences; the Soviet pilot flies the second prototype, with orders to wait for him at the North Cape area....
Answer:
Firefox