【16】There are four points $A, B, C, D$ arranged from left to right on a straight line. Using these four points as endpoints, 6 line segments can be formed. It is known that the lengths of these 6 line segments are $14, 21, 34, 35, 48, 69$ (unit: millimeters). Then the length of line segment $\mathrm{BC}$ is ( ) millimeters.
【Key Points】Figure counting, overlapping problems.
【Analysis】The longest side is $\mathrm{AD}=69$ millimeters,
since $34+35=69$ millimeters, $21+48=69$ millimeters, thus the overlapping part $\mathrm{BC}=14$ millimeters.

【Answer】14 millimeters