Question: Which song did Metallica release in 1991 from their fifth album that is one of their most well known?
Related documents: [0]  [DOC] Metallica (commonly known as The Black Album) is the fifth studio album by American heavy metal band Metallica. Released on August 12, 1991 by Elektra Records, it received widespread critical acclaim and became the band's best-selling album. "Metallica" produced five singles that are considered to be among the band's best-known songs, which include "Enter Sandman", "The Unforgiven", "Nothing Else Matters", "Wherever I May Roam", and "Sad but True". "Don't Tread on Me" was also issued to rock radio shortly after the album's release, but the song did not receive a commercial single release. The album marked a change in the band's sound from the thrash metal style of the previous four albums to a slower one. Metallica promoted the album with a series of tours. In 2003, the album was ranked number 252 on "Rolling Stone"' s 500 greatest albums of all time. [END] 
[1]  [DOC] "Enter Sandman" is a song by American heavy metal band Metallica. It was released as the first single from their eponymous fifth album, "Metallica" in 1991. The music was written by Kirk Hammett, James Hetfield and Lars Ulrich. Vocalist and rhythm guitarist Hetfield wrote the lyrics, which deal with the concept of a child's nightmares. [END] 
Answer: 
Enter Sandman
Enter Sandman