Task: Question Answering
Input (English): Passage: In February 1935, at Bridge's instigation, Britten was invited to a job interview by the BBC's director of music Adrian Boult and his assistant Edward Clark. Britten was not enthusiastic about the prospect of working full-time in the BBC music department and was relieved when what came out of the interview was an invitation to write the score for a documentary film, The King's Stamp, directed by Alberto Cavalcanti for the GPO Film Unit.
Britten became a member of the film unit's small group of regular contributors, another of whom was W. H. Auden. Together they worked on the documentary films Coal Face and Night Mail in 1935. They also collaborated on the song cycle Our Hunting Fathers (1936), radical both in politics and musical treatment, and subsequently other works including Cabaret Songs, On This Island, Paul Bunyan and Hymn to St. Cecilia. Auden was a considerable influence on Britten, encouraging him to widen his aesthetic, intellectual and political horizons, and also to come to terms with his homosexuality. Auden was, as David Matthews puts it, "cheerfully and guiltlessly promiscuous"; Britten, puritanical and conventional by nature, was sexually repressed.In the three years from 1935 to 1937 Britten wrote nearly 40 scores for the theatre, cinema and radio. Among the film music of the late 1930s Matthews singles out Night Mail and Love from a Stranger (1937); from the theatre music he selects for mention  The Ascent of F6 (1936), On the Frontier (1938) and Johnson Over Jordan (1939); and of the music for radio, King Arthur (1937) and The Sword in the Stone (1939).In 1937 there were two events of huge importance in Britten's life: his mother died, and he met the tenor Peter Pears. Although Britten was extraordinarily devoted to his mother and was devastated at her death, it also seems to have been something of a liberation for him. Only after that did he begin to engage in emotional relationships with people his own age or younger. Later in the year he got to know Pears while they were both helping to clear out the country cottage of a mutual friend who had died in an air crash. Pears quickly became Britten's musical inspiration and close (though for the moment platonic) friend. Britten's first work for him was composed within weeks of their meeting, a setting of Emily Brontë's poem, "A thousand gleaming fires", for tenor and strings.During 1937 Britten composed a Pacifist March to words by Ronald Duncan for the Peace Pledge Union, of which, as a pacifist, he had become an active member; the work was not a success and was soon withdrawn. The best known of his compositions from this period is probably Variations on a Theme of Frank Bridge for string orchestra, described by Matthews as the first of Britten's works to become a popular classic. It was a success in North America, with performances in Toronto, New York, Boston, Chicago and San Francisco, under conductors including John Barbirolli and Serge Koussevitzky. 
Question: What conductors performed  Variations on a Theme of Frank Bridge for string orchestra?
Output (English): Serge Koussevitzky.