Given the task definition and input, reply with output. In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: In 2001, President George W. Bush discussed an accord with Mexican President Vincente Fox. Possible accord was derailed by the September 11 attacks. From 2005 to 2013, the US Congress discussed various ways of controlling immigration. The Senate and House were unable to reach an agreement.Nearly 14 million immigrants entered the United States from 2000 to 2010, and over one million persons were naturalized as U.S. citizens in 2008. The per-country limit applies the same maximum on the number of visas to all countries regardless of their population and has therefore had the effect of significantly restricting immigration of persons born in populous nations such as Mexico, China, India, and the Philippinesthe leading countries of origin for legally admitted immigrants to the United States in 2013; nevertheless, China, India, and Mexico were the leading countries of origin for immigrants overall to the United States in 2013, regardless of legal status, according to a U.S. Census Bureau study. As of 2009, 66% of legal immigrants were admitted on the basis of family ties, along with 13% admitted for their employment skills and 17% for humanitarian reasons.Nearly 8 million people immigrated to the United States from 2000 to 2005; 3.7 million of them entered without papers. In 1986 president Ronald Reagan signed immigration reform that gave amnesty to 3 million undocumented immigrants in the country. Hispanic immigrants suffered job losses during the late-2000s recession, but since the recession's end in June 2009, immigrants posted a net gain of 656,000 jobs. Over 1 million immigrants were granted legal residence in 2011.For those who enter the US illegally across the MexicoUnited States border and elsewhere, migration is difficult, expensive and dangerous. Virtually all undocumented immigrants have no avenues for legal entry to the United States due to the restrictive legal limits on green cards, and lack of immigrant visas for low-skilled workers. Participants in debates on immigration in the early twenty-first century called for increasing enforcement of existing laws governing illegal immigration to the United States, building a barrier along some or all of the 2,000-mile (3,200 km) Mexico-U.S. border, or creating a new guest worker program. Through much of 2006 the country and Congress was immersed in a debate about these proposals. As of April 2010 few of these proposals had become law, though a partial border fence had been approved and subsequently canceled.
What two countries had a possible accord that was derailed by the September 11 attacks?