Question: Which singer is also a musician, Gwen Stefani or Charlie Simpson?

<P0-S0> Charles Robert "Charlie" Simpson (born 7 June 1985) is an English singer, songwriter and musician. <P0-S1>  He is the youngest member of multi-BRIT Award-winning band Busted, and is the lead vocalist, guitarist in post-hardcore band Fightstar. <P0-S2>  AllMusic has noted that Simpson is "perhaps the only pop star to make the convincing transition from fresh-faced boy bander to authentic hard rock frontman". <P0-S3>  Simpson is a multi-instrumentalist, playing guitar, keyboard, piano and drums. <SEP> <P1-S0> "What You Waiting For?" <P1-S1>  is a song by American singer Gwen Stefani from her debut solo studio album, "Love. <P1-S2>  Angel. <P1-S3>  Music. <P1-S4>  Baby. <P1-S5> " (2004). <P1-S6>  Written by Stefani and Linda Perry, the song is the album's opening track and was released as Stefani's debut solo single. <P1-S7>  "What You Waiting For?" <P1-S8>  details Stefani's lack of inspiration, fear of producing the album, as well as her reaction to pressures exerted by her record label. <P1-S9>  It is primarily an electropop song and introduces Stefani's four back-up dancers, the Harajuku Girls, who had a major input into the album's production. <SEP> <P2-S0>  After releasing five studio albums with ska punk group No Doubt, in 2004 Stefani began work as a solo artist, developing her pop debut "Love. <P2-S1>  Hip hop musician André 3000 made two appearances on the album, including on "Long Way to Go", and his alter ego "Johnny Vulture" on "Bubble Pop Electric". <P2-S2> ", "Rich Girl", and "Hollaback Girl", the latter of which was the first single to sell over a million digital copies in the United States. <P2-S3>  Baby. <P2-S4>  (The Remixes)" and included the Richard X remix of "Cool", which topped the United States Dance Club Songs in November 2005. <P2-S5> " in the same year. <P2-S6>  Angel. <P2-S7>  Music. <P2-S8> American singer and songwriter Gwen Stefani has recorded material for three studio albums and one extended play (EP), and has been featured on songs on other artists' respective albums. <P2-S9>  The record contained work from a variety of producers and songwriters, including Linda Perry, Nellee Hooper, André 3000, and No Doubt bandmate Tony Kanal. " <P2-S10>  Angel. <P2-S11> Cool", "Luxurious", and "Crash" were also released as the singles. <P2-S12>  Baby. <P2-S13>  It produced several top ten singles worldwide, including "What You Waiting For? <P2-S14>  Music. <SEP> <P3-S0> Gwen Renée Stefani ( ; born October 3, 1969) is an American singer, songwriter, fashion designer, actress, and television personality. <P3-S1>  She is a co-founder and the lead vocalist of the band No Doubt that experienced major success after their breakthrough studio album "Tragic Kingdom" (1995) along with various successful singles, including "Just a Girl", "Don't Speak", "Hey Baby", and "It's My Life". <P3-S2>  During the band's hiatus, Stefani embarked on a solo pop career in 2004 by releasing her debut studio album "Love. <P3-S3>  Angel. <P3-S4>  Music. <P3-S5>  Baby. <P3-S6> " Inspired by pop music from the 1980s, the album was met with both critical and commercial success. <P3-S7>  It spawned three commercially successful singles: "What You Waiting For? <P3-S8> ", "Rich Girl", and "Hollaback Girl", the latter reached number one on the "Billboard" Hot 100 while also becoming the first US download to sell one million copies. <P3-S9>  In 2006 Stefani released her second studio album "The Sweet Escape". <P3-S10>  The album produced two successful singles: "Wind It Up" and the album's title track "The Sweet Escape". <P3-S11>  Her third solo album "This Is What the Truth Feels Like" was released in March 2016 and became her first solo number-one album on the "Billboard" 200.