Which secretary of defense was kept in office by President Obama after serving under George W. Bush?

In 1913, Elmer McCollum helped discover which nutrient, needed for healthy eyes? By mid-1990, all of Liberia except parts of which capital city was controlled by rival rebel groups? After defeating his brothers in a civil war, Mehmed I reunited which empire in 1413? Motorola's StarTAC, introduced in 1996, was an early flip-style version of what device? By occupation, Antonin Scalia and William Rehnquist were what? Which humorist died with aviator Wiley Post in a plane crash near Point Barrow, Alaska, on August 15, 1935? By population, which is the largest city in Canada? What is the term for a compiler of dictionaries, such as Noah Webster?