An open three can be capped at either end, so the geometry allows two blocks — but they are not equal. The right end is the one YOUR stones can reach: a block that also connects builds your counterplay while it defends; a block in the wilderness defends one threat and builds nothing. Here white's row three is live, and black already owns a stone on the row's far horizon. Cap the end that talks to it.
Solve the remaining tasks to finish this lesson.