单选题 ()is a line between two vertices in a digraph.

A node
B. An arc
C. An edge
D. A path

单选题 A vertex in a digraph has four arcs entering and three arcs leaving. The outdegree of the vertex is ().

A. 4
B. 3
C. 2
D. 1

单选题 A vertex in a digraph has 2 arcs entering and 3 arcs leaving. The degree of the vertex is ().

A. 2
B. 3
C. 4
D. 5

单选题 A (n) ____list is also known as a queue.

A. LIFO
B. FIFO
C. unordered
D. ordered

单选题 A (n) ____ list is also known as a stack.

A. LIFO
B. FIFO
C. unordered
D. ordered

单选题 If A is the first data element input into a stack followed by B, C, and D, （ ）is the first element to be removed.

A
B
C
D

If A is the first data element input into a queue followed by B, C, and D, （）is the first element to be removed.

A
B
C
D

填空题 If the tree is not empty, the first node is called ____, which has the indegree of ____.

填空题 All nodes in the tree, except ____, must have an indegreee of ____.

填空题 A ____in a tree is a node with an outdegree of zero.