Q: You are given the conttent of a table in the input (row by row). Generate an unambiguous question based on the information present in the table, such that: 1) It's answer is unique and is in the given table, 2) Its answer is a contiguous text span from the table. Avoid creating questions that can be answered correctly without reading/understanding the table
Table: ['Category', 'Speed (km/h)', 'Speed (mph)', 'Vehicle', 'Pilot', 'Date']. ['Rocket-powered aircraft', '7,258', '4,510', 'North American X-15', 'William J. Knight', '3 Oct 1967']. ['Manned air-breathing craft', '3,530', '2,194', 'Lockheed SR-71 Blackbird', 'Eldon W. Joersz', '28 Jul 1976']. ['Propeller-driven aircraft', '870', '541', 'Tupolev Tu-114', 'Ivan Soukhomline', '1960']. ['Piston-engined propeller-driven aircraft', '850.1', '528.33', 'Grumman F8F Bearcat Rare Bear (N777L)', 'Lyle Shelton', '21 Aug 1989']. ['Helicopter', '401.0', '249.1', 'Westland Lynx 800 G-LYNX', 'John Egginton', '11 Aug 1986']. ['Glider (sailplane)', '306.8', '190.6', 'Schempp-Hirth Nimbus-4DM', 'Klaus Ohlmann and Matias Garcia Mazzaro', '22 Dec 2006']. ['Human-powered aircraft', '32', '19.8', 'MIT Monarch B', 'Frank Scarabino', '1 May 1984']
A:
which airplanes had a speed under 600 miles per hour?