 Complete the following phrase:  Offenbachs career was at its height in the 1860s with the premieres of some of his most popular and enduring works, such as La Belle Hélène 1864 and La Vie parisienne 1866. With the original production of the latter still running, Offenbach and his librettists hurried to prepare a new opera, La Grande-Duchesse de Gérolstein, to play during the Paris Exposition 