Read this: The headquarters of the MoD are in Whitehall and are now known as Main Building. This structure is neoclassical in style and was originally built between 1938 and 1959 to designs by Vincent Harris to house the Air Ministry and the Board of Trade. The northern entrance in Horse Guards Avenue is flanked by two monumental statues, Earth and Water, by Charles Wheeler. Opposite stands the Gurkha Monument, sculpted by Philip Jackson and unveiled in 1997 by Queen Elizabeth II. Within it is the Victoria Cross and George Cross Memorial, and nearby are memorials to the Fleet Air Arm and RAF (to its east, facing the riverside). A major refurbishment of the building was completed under a PFI contract by Skanska in 2004.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): When was Horse Guards Avenue unveiled by Queen Elizabeth II?
unanswerable