You will be given a definition of a task first, then some input of the task.
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: "What You Waiting For?" was released as the lead single from Love. Angel. Music. Baby. on September 28, 2004. The single peaked at number 47 on the Billboard Hot 100, and was commercially successful overseas, topping the chart in Australia and reaching the top 10 in several countries including France, Netherlands, New Zealand, Sweden, and the United Kingdom. "Rich Girl", featuring Eve, was released as the album's second single on December 14, 2004, becoming Stefani's first top-10 entry as a solo artist in the US when it peaked at number seven on the Billboard Hot 100. Elsewhere, the song performed equally as successfully as "What You Waiting For?". "Hollaback Girl" was released as the third single on March 15, 2005. It became the album's best-selling and most popular single, while also becoming the first single to sell one million digital copies in the US. The song topped the Billboard Hot 100 within six weeks of its release, earning Stefani her first number-one single on the chart."Cool" was released as the fourth single from the album on July 5, 2005. The song fared moderately on the charts, reaching the top 10 in Australia and New Zealand, as well as the top 20 in Denmark, Germany, Ireland, Norway, the UK and the US. "Luxurious" was released as the fifth single in October 2005. The single version features rapper Slim Thug. The song was less successful than the previous singles from the album, peaking at number 21 on the Billboard Hot 100. "Crash" was not originally planned as a single, but due to Stefani's pregnancy, her second solo album was delayed, and the song was released as the sixth and final single from the album on January 24, 2006.
Output:
What is the title of the song that peaked at number 21 on the Billboard Hot 100?