The correct answer is A, C, D only Key Points Complementary Events: Complementary events are two occurrences that occur only if and only if the other does not occur. Two events must be mutually exclusive and exhaustive in order to be characterised as complimentary. Important Points The sum of probabilities of complementary events must be equal to 1. P(A) + P(A') = 1 Complementary events can take place only when there are exactly two outcomes. The complement and the event are mutually exclusive. This signifies that no results are shared between the event and its complement. An event and its complement are also collective exhaustive. This signifies that the event and its complement contain all the sample space's outcomes.