When three line segments with the lengths of $3$, $$4$$, and $$7$$ are used to form a triangle, it is called a/an~\uline{~~~~~~~~~~}~. 
Overseas Competition->Knowledge Point->Counting Modules->Statistics and Probability
∵$$3+4=7$$,  ∴ Three line segments measuring $$3$$, $$4$$, and $$7$$ cannot form a triangle,  ∴ Using three line segments measuring $$3$$, $$4$$, and $$7$$ to form a triangle is an impossible event.  So $$\text{C}$$ is the answer. 