Question: Who is Bruce Spizer an expert on, known as the most influential act of the rock era?

Context:
The Beatles
The Beatles were an English rock band formed in Liverpool in 1960.
 With members John Lennon, Paul McCartney, George Harrison and Ringo Starr, they became widely regarded as the foremost and most influential act of the rock era.
 Rooted in skiffle, beat and 1950s rock and roll, the Beatles later experimented with several musical styles, ranging from pop ballads and Indian music to psychedelia and hard rock, often incorporating classical elements and unconventional recording techniques in innovative ways.
 In 1963 their enormous popularity first emerged as "Beatlemania", and as the group's music grew in sophistication in subsequent years, led by primary songwriters Lennon and McCartney, they came to be perceived as an embodiment of the ideals shared by the counterculture of the 1960s.
Marsha Albert
Marsha Albert (born 1948) is credited with being the person who jump-started the early 1960s phenomena known as Beatlemania in the United States when as a 15-year-old girl, on 17 December 1963, she introduced for the first time on American radio a song written and recorded by The Beatles titled I Want to Hold Your Hand (the Beatles' best-selling single worldwide), and that Beatles historian and author Bruce Spizer noted, in 2004, by his stating "Marsha Albert's actions forced a major record company to push up the release date of a debut single from an unknown band during the holiday season, a time when record companies traditionally released no new product."
Cargo Records (Canada)
Cargo Records was a Canadian independent record label and distributor, active in the 1980s and 1990s.
 Based in Montreal, the company both released albums directly as a label, and distributed albums on behalf of many other small independent labels, making it one of the largest and most influential Canadian record companies of the alternative rock era.
Azra
Azra was a rock band that was popular across Yugoslavia in the 1980s.
 Azra was formed in 1977 by its frontman Branimir "Johnny" Štulić.
 The other two members of the original line-up were Mišo Hrnjak (bass) and Boris Leiner (drums).
 The band is named after a verse from "Der Asra" by Heinrich Heine.
 They are considered to be one of the most influential bands from the former Yugoslav new wave rock era and the Yugoslav rock scene in general.
The Beach Boys
The Beach Boys are an American rock band formed in Hawthorne, California, in 1961.
 The group's original lineup consisted of brothers Brian, Dennis, and Carl Wilson; their cousin Mike Love; and their friend Al Jardine.
 Distinguished by their vocal harmonies and early surf songs, they are one of the most influential acts of the rock era.
 The group, led by their principal songwriter and producer Brian, pioneered novel approaches to popular music form and production, combining their affinities for jazz-based vocal groups, 1950s rock and roll, and black R&B to create their unique sound.
 He later arranged his compositions for studio orchestras and explored a variety of other styles, often incorporating classical or jazz elements and unconventional recording techniques in innovative ways.
Christian Death
Christian Death is an American deathrock band formed in Los Angeles, California, in 1979 by Rozz Williams.
 They became a highly influential act, heralding the American gothic rock and deathrock movements.
Cultural impact of the Beatles
The Beatles were an English rock band, formed in Liverpool in 1960.
 With members John Lennon, Paul McCartney, George Harrison and Ringo Starr, they became widely regarded as the foremost and most influential act of the rock era.
 In the early 1960s, their enormous popularity first emerged as "Beatlemania", but as the group's music grew in sophistication, led by primary songwriters Lennon and McCartney, they came to be perceived as an embodiment of the ideals shared by the counterculture of the 1960s.
Adam and the Ants
Adam and the Ants were an English rock band active during the late 1970s and early 1980s.
 The group, which lasted from 1977 to 1982, existed in two incarnations, both fronted by Adam Ant.
 The first, founded in May 1977 and known simply as The Ants until November that year, achieved considerable cult popularity during the transition from the punk rock era to the post-punk and new wave era and were noted for their high camp and overtly sexualised stage performances and songs.
 The final line-up of this first incarnation – Dave Barbarossa, Matthew Ashman and Leigh Gorman – left the band in January 1980 at the suggestion of then-de facto manager Malcolm McLaren, to form the instrumentalist personnel of the controversial Bow Wow Wow.
Bruce Spizer
David "Bruce" Spizer (born July 2, 1955) is a tax attorney in New Orleans, Louisiana, who is also recognized as an expert on The Beatles.
 He has published eight books, and is frequently quoted as an authority on the history of the band and its recordings.
Cultural depictions of Elvis Presley
Elvis Presley has inspired artistic and cultural works since he entered the national consciousness.
 From that point, interest in his personal and public life has never stopped.
 Some scholars have even studied many aspects of his profound cultural influence. "
Billboard" historian Joel Whitburn declared Presley the "#1 act of the Rock era".

Answer: The Beatles