10. The diagram below shows five rectangles, each containing some of the letters $\mathrm{P}, \mathrm{R}, \mathrm{I}, \mathrm{S}$ and $\mathrm{M}$. Harry wants to cross out letters so that each rectangle contains only one letter and each rectangle contains a different letter. Which letter does he not cross out in rectangle 2 ?
A P
B R
C I
D S
E M
10. B Rectangle 4 only contains one letter and hence letter $S$ must be crossed out in any other rectangle. Therefore letter $\mathrm{P}$ is the only letter left in rectangle 1 and must be crossed out in all the other rectangles. This means letter $\mathrm{I}$ is the only one left in rectangle 3 and must be crossed out in all other rectangles. This leaves letter $\mathrm{R}$ not crossed out in rectangle 2 .