Nathan Donald "Nate" Diaz (born April 16, 1985) is an American professional mixed martial artist currently competing for the Ultimate Fighting Championship (UFC).
 He was the winner of "The Ultimate Fighter 5".
 Diaz is the younger brother of former Strikeforce Welterweight Champion and WEC Welterweight Champion, Nick Diaz.
 Prior to signing with the UFC, Diaz competed in World Extreme Cagefighting, Strikeforce, and Pancrase.
 As of June 6th 2017, Diaz is #8 in official UFC lightweight rankings.
WEC 38: Varner vs. Cerrone was a mixed martial arts event held by World Extreme Cagefighting on January 25, 2009.
 It aired live on the Versus Network.
 In the main event, WEC Lightweight Champion Jamie Varner defended his title against undefeated top contender Donald Cerrone.
Christian Cyrus Leben (born July 21, 1980) is a retired American mixed martial artist.
 He formerly competed with the UFC as a Middleweight, compiling a record of 12-10 in 22 appearances for the organization.
 Leben first appeared in the inaugural season of "The Ultimate Fighter" reality series, and has also fought in the now-defunct WEC.
 He began his UFC career with five victories in a row before suffering a knockout loss to future UFC Middleweight Champion Anderson Silva.
 Leben was the inaugural WEC Middleweight Champion.
Michael Bisping ( , ; born 28 February 1979) is an English mixed martial artist and actor.
 He fights in the Ultimate Fighting Championship (UFC), and is the current UFC Middleweight Champion.
 He is a former Cage Rage Light Heavyweight Champion, and "The Ultimate Fighter 3" Light Heavyweight Tournament winner.
 At UFC 78, he became the first English fighter in a UFC main event and at UFC 199, he became the first English fighter to win a UFC championship.
Anthony Paul Pettis (born January 27, 1987) is an American mixed martial artist currently competing as a Lightweight in the Ultimate Fighting Championship (UFC), where he is the former UFC Lightweight Champion.
 Pettis was also the final WEC Lightweight Champion prior to the promotion being merged into the UFC.
 At one point, he was ranked as a top-ten pound for pound fighter.
 He is currently ranked #13 in the official UFC lightweight rankings.
The Khalid Bin Hamad Al Khalifa Mixed Martial Arts (KHK MMA) is a martial arts organization based in Bahrain.
 It was founded under the patronage of His Highness Khalid bin Hamad Al Khalifa for its current CEO and professional MMA fighter Mohammed "The Hawk" Shahid in February 2015.
 Fighters competing under KHK MMA banner include: former UFC lightweight champion Frankie Edgar, UFC lightweight contender Khabib Nurmagomedov, UFC lightweight fighter Islam Makhachev, Bellator fighter James Gallagher, 2-time IMMAF champion Jose "Shorty" Torres, former UFC fighter and current Titan FC contender Alex Soto, and current Desert Force fighter Hamza Kooheji.
 KHK MMA activities include: coaching pro fighters, developing amateur MMA, promoting martial arts events, fitness gyms, and a sports gear brand.
Frank Shamrock (born Frank Alisio Juarez, III; December 8, 1972) is an American former mixed martial artist.
 Shamrock was the first to hold the UFC Middleweight Championship (later renamed the UFC Light Heavyweight Championship) and retired as the four-time defending undefeated champion.
 Shamrock was the No. 1 ranked pound for pound UFC fighter in the world during his reign as the UFC Middleweight Champion.
 Shamrock has won numerous titles in other martial arts organizations, including the interim King of Pancrase title, the WEC Light Heavyweight Championship and the Strikeforce Middleweight Championship.
The year 2009 is the 9th year in the history of World Extreme Cagefighting, a mixed martial arts promotion based in the United States.
 In 2009 WEC held 8 events beginning with, "WEC 38: Varner vs. Cerrone".
José Aldo da Silva Oliveira Junior (] ; born September 9, 1986) is a Brazilian mixed martial artist in the Ultimate Fighting Championship.
 He was the fourth and final WEC Featherweight Champion and thus, became the first UFC Featherweight Champion during the UFC/WEC merger.
 He is a former two-time UFC Featherweight Champion.
 He was named Sherdog's 2009 Fighter of the Year.
 He is currently #12 in official UFC pound-for-pound rankings, having been ranked as high as #1 in 2015, and ranked the #2 featherweight in the world and #7 pound-for-pound by Sherdog.
 In the decade from November 2005 through December 2015, Aldo was undefeated in 18 fights.
 In Sherdog's April 2017 Pound-For-Pound ranking, Aldo was called "the greatest featherweight in mixed martial arts history."
Donald Anthony Cerrone ( ; born March 29, 1983) is an American professional mixed martial artist and former professional kickboxer currently competing in the UFC's Lightweight and Welterweight divisions.
 He is ranked #6 in the official UFC Welterweight rankings tied with fellow Welterweight contender Neil Magny.
 A professional competitor since 2006, Cerrone has also formerly competed for the WEC.
 Cerrone currently holds the record for most post-fight bonuses (Fight/Performance of the Night, Knockout/Submissions of the Night) in UFC/WEC history with 18.
 He is also known as the "busiest" fighter in the UFC, due to consistently having at least four fights per year since 2013.

Question: Which fighter from WEC 38 is known as the "busiest" fighter in the UFC?
Answer: Donald Cerrone