选项A 表述正确。选项B 本身表述错误；根据Depression rates are higher in countries that place a premium on happiness 可知幸福感越高的国家抑郁也严重，并非所有国家抑郁率高于幸福感，选项C 表述过于片面；根据Rather than being the by-product of a life well-lived 可知快乐不仅仅是幸福生活的副产品，选项D 表述片面。故正确的答案是选项A。23. How long were these volunteers being tracked? (