Select the correct choice that completes the sentence below. For any event $A, P(A)+P(\operatorname{not} A)=$ 0.5 . 2. 1. 0.

Step 1 ：The question is asking for the sum of the probability of an event A and the probability of not A. In probability theory, the sum of the probabilities of an event and its complement (not A) is always equal to 1. This is a fundamental rule of probability.

Step 2 ：Final Answer: \(\boxed{1}\)