There are Mormons on Yahoo Answers who never answer Mormon Questions, and there are some who answer both LDS questions and who answer other Non-LDS related questions, and
there are still other Latter Day Saint answerers who only answer LDS related questions on Yahoo Answers.
Most legit Latter Day Saints have their previous questions and answers and their network public.
If they don't, if it is Private and they claim they are LDS, then that is a Red Flag. This does not mean that they are not a Mormon, but having a Private Yahoo Answers Account is definitely a sign that someone is a Troll possibly with bad intentions (perhaps to defame or to lead others astray using deception as the means of doing that).
So a good idea is to look at a users previous questions, and/or look at who they are Fans of or who is Fans of them.
Really hard to say how many Yahoo Answerers are LDS. There are about a dozen Yahoo Answerers that answer the bulk of the LDS related questions on Yahoo Answers.
After that there are maybe thirty three more who chip in from time to time and put their input in.
A) 21 B) 19 C) About five years, more or less.
http://www.youtube.com/watch?v=Anq1eBQgiR0
http://www.youtube.com/watch?v=CajagruS0h8
http://Mormon.org Chat.
God bless.