The maximum sum is $$6+6=12$$, so "the sum of dots is $$13$$" is an impossible event.  So $$\text{D}$$ is the answer. 
Throw two dice of the same quality and size. The six sides of each die are marked with number of dots from $$1$$ to $$6$$, respectively. Among the following options,~\uline{~~~~~~~~~~}~is an impossible event. 
a. The sum of dots is $$12$$. 
b. The sum of dots is smaller than $$3$$. 
c. The sum of dots is larger than $$4$$ but smaller than $$8$$. 
d. The sum of dots is $$13$$. 