To Change the Closure Setting for a B-spline Curve

Select the Change Curve Closure tool ( Modeling > Curves > Curve Utilities > Change Curve > Closure ).
Select the B-spline curve to edit.
The current closure setting for the B-spline curve is displayed, with the reverse setting shown in the option menu.
Enter a data point to accept the change.

Changing Closure Settings for B-Spline Curves. Left = Closed Curve, Right = Open Curve