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: What kind of weapon can shoot down a fully loaded passenger jet flying at nearly 33,000 feet? That could be a key question in the Malaysia Airlines Flight 17 crash investigation, as officials try to determine what was responsible for Thursday's plane crash in volatile eastern Ukraine. A radar system saw a surface-to-air missile system turn on and track an aircraft right before the plane went down, a senior U.S. official told CNN's Barbara Starr. And a second system saw a heat signature at the time the airliner was hit. The United States is analyzing the trajectory of the missile to pinpoint where the attack came from, the official said.Senior U.S. official: Radar saw surface-to-air missile system track aircraftA sophisticated surface-to-air missile was more likely used, experts sayOne possibility is the Russian-made Buk missile system, retired Army officer saysThe sophisticated weapon would require professional soldiers, expert says 
 Question:Such weapons travel with _ troops at the division level, Francona said.

Russian