Passage: Following Music Box, Carey recorded a duet with Luther Vandross; a cover of Lionel Richie and Diana Ross's "Endless Love", and began working on an unknown project throughout the summer of 1994. In October, Billboard announced that she would release a holiday album later that year.  Carey's album Merry Christmas was released on November 1, 1994 and eventually became one of the best-selling Christmas albums of all time, with global sales of over 15 million copies. Upon its release, the album's first single, "All I Want for Christmas Is You", was described as a "holiday standard" and considered "one of the few worthy modern additions to the holiday canon." Commercially, it became the world's 10th best-selling single by December 2018, with global sales exceeding 16 million copies. By the end of the holiday season of 1994, Carey and Afanasieff had already begun writing material for her next studio album which would be released late the following year.Released on October 3, 1995, Daydream combined the pop sensibilities of Music Box with downbeat R&B and hip hop influences. Critically, the album was heralded as Carey's best to date; The New York Times named it one of 1995's best albums, and wrote, "best cuts bring R&B candy-making to a new peak of textural refinement [...] Carey's songwriting has taken a leap forward and become more relaxed, sexier and less reliant on thudding clichés." The album's second single, "One Sweet Day", a collaboration with R&B group Boyz II Men, remained atop the Billboard Hot 100 for a record-breaking 16 consecutive weeks, becoming the longest-running number-one song in history. Daydream became her biggest-selling album in the United States, and became her second album to be certified Diamond by the RIAA, following Music Box. The album sold 2.2 million copies in Japan alone and eventually reached global sales of over 25 million copies.Due to the album's success, Daydream and its singles were respectively nominated in six categories at the 38th Grammy Awards. Carey, along with Boyz II...

Question: what becames carey's biggest selling album in the US?
Daydream