1. Which of the following commands can be used to create a new directory in a Linux system terminal:

{{ select(1) }}

• newdir
• mkdir
• create
• mkfold

2. (Multiple Answers) Which item in the following connected undirected graph may NOT be always able to be colored with no more than two colors:

{{ multiselect(2) }}

• A complete trident tree
• A plan
• An edge-biconnected graph
• An eulerian

3. Assuming there are $n$ columns, circular rings numbered $1,2,3,\ldots$ need to be placed in sequence according to the following rules: the bottom of each column is fixed, and a circular ring can be placed at the top; Every time a circular ring is inserted from the top of the column, it is necessary to ensure that the sum of the numbers of any two adjacent circular rings is a perfect square number. Please calculate that when there are $4$ pillars, a maximum of {{ input(3) }} rings can be placed.