Which command is used to combine two lines into a single segment?

The command that is used to combine two lines into a single segment is the JOIN command. This functionality is particularly useful in AutoCAD for cleaning up drawing objects and creating cleaner geometries. When two lines are joined using this command, they merge into one continuous line, which can help streamline the design process and improve the overall organization of the drawing.

The JOIN command works effectively with lines, polylines, arcs, and other geometric entities, allowing for a seamless integration of shapes. It is not limited to just straight lines, making it quite versatile for different types of geometry in the design.

The other choices represent commands that serve different purposes. MERGE is typically not recognized as a standard AutoCAD command for line segments, and while UNITE is a command used in solid modeling to combine 3D objects, it does not apply to 2D line segments. CONNECT, often associated with connecting objects in some contexts, does not specifically refer to the merging of lines into a single segment in AutoCAD. Thus, the JOIN command is the appropriate choice for this action within the software.