Read the sentence from The Woman in White. Finding us distinguished, as a nation, by our love of athletic exercises, the little man, in the innocence of his heart, devoted himself impromptu to all our English sports and pastimes whenever he had the opportunity of joining them; firmly persuaded that he could adopt our national amusements of the field by an effort of will precisely as he had adopted our national gaiters and our national white hat. Based on the context in which the word impromptu is used, which response most accurately defines this word?