A railway network links some towns in a country such that tracks lie between M and N, between M and O, between O and R, between R and T, between R and U, between T and P and between P and S. No two tracks intersect each other except at the towns. A train can travel from P to N only in that direction and not back. For a train to go from S to N, it is necessary to go through town:

• A. M
• B. P
• C. R
• D. T