21. Samantha's house is two blocks west and one block south of the southwest corner of City Park, and her school is two blocks east and two blocks north of the northeast corner of City Park. On her way to school, Samantha always rides her bike to the southwest corner of City Park, then cuts diagonally through the park to the northeast corner, and continues to her school. To make Samantha's bike route as short as possible, there are ( ) different routes she can take to school.
(A) 3
(B) 6
(C) 9
(D) 12
(E) 18
21. E.

Samantha needs to walk a minimum of three blocks from her home to the southwest corner of the park: she walks one block east twice and one block north once, which gives her three possible routes; she needs to walk a minimum of four blocks from the northeast corner of the park to her school: she walks two blocks east and two blocks north, which gives her six possible routes.
Therefore, the entire journey has $3 \times 6=18$ different routes.