9. The diagram on the right shows the positions of four people (each marked $\times$ ) in an Art Gallery. In the middle of the room is a stone column. Ali can see none of the other three people. Bea can see only Caz. Caz can see Bea and Dan. Dan can only see Caz.
Who is at position $P$ ?

A Ali
B Bea
C $\mathrm{Caz}$
D Dan
E More information needed.
Solution: $\mathbf{C}$
The person in position $P$ is the only one who can see two of the others. We are told that Caz can see Bea and Dan but everyone else can see just one other person or no-one. So it must be $\mathrm{Caz}$ who is at position $P$.
(We can also see that Ali, who can see no-one, must be in the position shown. Bea and Dan must be in the other two positions, but the information we are given doesn't enable us to work out which is where.)