In an article, "Preparing for the Next Black Swan" (Wall Street Journal, Aug 21, 2010), the point is made that diversification may be insufficient in protecting one's portfolio during a "Black Swan" event. Why may this be true?

A) virtually all asset classes may decline at the same time
B) investors may be unable to buy different assets during a "Black Swan" event
C) some assets may rise while others decline during a "Black Swan" event
D) Black Swan events are surprises and thus one cannot prepare for such an event.

A