The area is 18 units squared, I believe.
A is the correct answer.
By definition, a function is a relation where each input has exactly one output. In letter A, the inputs 0, 1, 2, and 3 all have exactly one output.
In B, the input 1 has two outputs.
In C, the input 2 has two outputs.
In D, the input 2 also has two outputs.
Well, if the notebooks cost x, then you know that
3x + 45.87 = 60.54
right? so, solve for x. 60.54 - 45.87 = 14.67
Divide the difference by 3.