The correct answer to this question is letter "a) region 1, given that the search of region 1 was unsuccessful." A plane is missing and is presumed to have equal probability of going down in any of three regions. If a plane is actually down in region i, let 1-αi denote the probability that the plane will be found upon search of the ith region, i=1,2,3. The conditional probability that the plane is in region 1, given that the search of region 1 was unsuccessful.