is not a random variable---it only takes one value.
Answer: Our knowledge of 
can be considered as a random variable.
? We will discuss this so called a priori knowledge
of B later on---it essentially summarizes all previous (other) knowledge of
gained from previous (other) experiments.
? The probability of measurement of 
given any possible 
.
Nobody knows what this is!
You essentially determine it by normalizing 
or equivalently note that relative probabilities
are indeed independent of the noxious 
.