The ornamental design for a ball-point pen, as shown and described.
1. Ball-point pen
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Left
1.5 : Right
1.6 : Top
1.7 : Bottom
The portions of the design represented by broken lines are disclaimed; the present design is created on the basis of chameleon's tail; the tail of the ball-point pen is made of elastic material.