What is ROE?

According to Google “define ROE”:

“The mass of eggs contained in the ovaries of a female fish or shellfish, typically including the ovaries themselves…”

So my question is… which one of these is the correct definition?

A. ROE(2011) = Net Income(2011) / Book Value(2011)

B. ROE(2011) = Net Income(2011) / Book Value(2010)

C. Other: __________

I’ve done some practice questions (CFAI) and have seen both of these definitions used in answers. So I’m confused about which one is right. Personally, I think B makes more sense. But A has definitely appeared in many places, such as CFAI Mock 2011 (PM session, Question 49).

i remember the denominator is the average of book value of 2011 and 2010.

It will either be B or what Dingying85 stated which is the average of the most recent two periods. The questions I’ve seen will sometimes tell you to just use the most recent BV otherwise you would assume average.