When only a part of a surface is to be designated as a datum, a ____ line drawn parallel to the surface is used to indicate the portion of the surface acting as the datum.

A) center
B) chain
C) visible
D) cutting-plane

A chain line, characterized by a series of dashes and two dots, is used to indicate a specific portion of a surface that serves as a datum when only a part of the surface is to be designated as such. This helps in specifying the exact area of interest on technical drawings.