High School

Choose the expression that represents a linear expression.

A. [tex]−17x^4 − 18x^3 + 19x^2 − 20x + 21[/tex]
B. [tex]18x^3 + 19x^2 − 20x + 21[/tex]
C. [tex]23x^2 + 24x − 25[/tex]
D. [tex]4x + 4[/tex]