Task: Question Answering
Input (English): Passage: In 1848, at the age of thirteen, Saint-Saëns was admitted to the Paris Conservatoire, France's foremost music academy. The director, Daniel Auber, had succeeded Luigi Cherubini in 1842, and brought a more relaxed regime than that of his martinet predecessor, though the curriculum remained conservative. Students, even outstanding pianists like Saint-Saëns, were encouraged to specialise in organ studies, because a career as a church organist was seen to offer more opportunities than that of a solo pianist. His organ professor was François Benoist, whom Saint-Saëns considered a mediocre organist but a first-rate teacher; his pupils included Adolphe Adam, César Franck, Charles Alkan, Louis Lefébure-Wély and Georges Bizet. In 1851 Saint-Saëns won the Conservatoire's top prize for organists, and in the same year he began formal composition studies. His professor was a protégé of Cherubini, Fromental Halévy, whose pupils included Charles Gounod and Bizet.Saint-Saëns's student compositions included a symphony in A major (1850) and a choral piece, Les Djinns (1850), after an eponymous poem by Victor Hugo. He competed for France's premier musical award, the Prix de Rome, in 1852 but was unsuccessful. Auber believed that the prize should have gone to Saint-Saëns, considering him to have more promise than the winner, Léonce Cohen, who made little mark during the rest of his career. In the same year Saint-Saëns had greater success in a competition organised by the Société Sainte-Cécile, Paris, with his Ode à Sainte-Cécile, for which the judges unanimously voted him the first prize. The first piece the composer acknowledged as a mature work and gave an opus number was Trois Morceaux for harmonium (1852). 
Question: What is the name of the first piece Saint-Saëns acknowledged as a mature work?
Output (English): Trois Morceaux for harmonium.