Mathematical symbols representing probability events

A number of multiple choice questions which I am working on relate to mathematical symbols which represent probability events…in order to tackle the questions, I need to be clear what is/ is not a valid mathematical symbol…. Are a, b, c, d (see below) valid mathematical symbols which can be used to represent a probability event? I think a, b and c are valid symbols, but am not sure about d - can a single letter be used to represent a probability event? I would appreciate any clarification on these symbols i.e. confirmation they are/ are not valid probability symbols. Thanks in advance.

a) S ∩ R (intersection)

b) S ∪ R (union)

c) ~S (complement)

d) S

NB

S stands for swimming, R stands for running

Single letter as mathematical symbol in probability

Thanks very much. I just want to be sure that a **single letter** is valid as a mathematical symbol in probability – this is not made clear in the textbook I am using and I have not been able to locate any internet resources which clarify this. Could you advise as to whether the following list of definitions is correct? I know the first 3 are correct, I just need clarification on the validity of the single letters – I am probably overthinking this but just wanted to doublecheck!

S ∩ D (intersection) represents the probability of swimming and running

S ∪ D (union) represents the probability of swimming or running

~S (complement) represents the probability of not swimming

S simply represents the probability of swimming

R simply represents the probability of running