Question of the Day: Prince Rainier III of Monaco married which American actress in April 1956?

Prince Rainier III of Monaco married which American actress in April 1956?

Prince Rainier III (Rainier Louis Henri Maxence Bertrand Grimaldi; 31 May 1923 – 6 April 2005) ruled the Principality of Monaco for almost 56 years, making him one of the longest ruling monarchs in European history.