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: Washington (CNN) The advertised purpose of President Barack Obama's pre-vacation news conference Thursday was to highlight gains in the war against ISIS in Iraq and Syria with the Pentagon as a backdrop. But for all his talk of terrorism, Russia and other geopolitical challenges, the underlying argument he wanted to make was clear: it's a serious business being commander-in-chief and the Republican who wants his job isn't fit for the Oval Office. Obama ridiculed Donald Trump's recent suggestion that the election system could be rigged, called on the candidate to act like a president since he's soon to be briefed on confidential information and implied that he didn't believe the billionaire businessman could be trusted with America's nuclear codes.Obama hit out at Trump just two days after branding him unfit to be commander in chiefTrump and Clinton are expected to soon start getting classified intelligence briefings 
 Question:Obama pointed to many such challenges in Thursday's _ news conference, particularly the fight against ISIS, also known as ISIL.

Pentagon