单选题：Assume that you are a real world Chinese postman, which have lea

Assume that you are a real world Chinese postman, which have learned an awesome course "Advanced Data Structures and Algorithm Analysis" (ADS). Given a 2-dimensional map indicating $$N$$ positions $$p_i(x_i,y_i)$$ of your post office and all the addresses you must visit, you'd like to find a shortest path starting and finishing both at your post office, and visit all the addresses at least once in the circuit. Fortunately, you have a magic item "Bamboo copter & Hopter" from "Doraemon", which makes sure that you can fly between two positions using the directed distance (or displacement).

("Bamboo copter & Hopter", japan12.com/bamboo-copter-hopter)

However, reviewing the knowledge in the ADS course, it is an $$NPC$$ problem! Wasting too much time in finding the shortest path is unwise, so you decide to design a $$2-approximation$$ algorithm as follows, to achieve an acceptable solution.
```
Compute a minimum spanning tree T connecting all the addresses.
Regard the post office as the root of T.
Start at the post office.
Visit the addresses in order of a _____ of T.
Finish at the post office.
```
There are several methods of traversal which can be filled in the blank of the above algorithm. Assume that $$P\neq NP$$, how many methods of traversal listed below can fulfill the requirement? @[C](4)
* Level-Order Traversal
* Pre-Order Traversal
* Post-Order Traversal

A. 0
B. 1
C. 2
D. 3




A.0
B.1
C.2
D.3


答案：C