Simple array problem

[Giovanni Di Maria via Digitalmars-d-learn]

Hi.
I am very new to D Language and I apologize for my bad question.

Why the followin code gives me the error?

 Error: only one index allowed to index `int[3][3]`

import std.stdio;
import std.array;
void main()
{
    int[3][3] matrix;
        matrix[2,2]=99;
}


Thank you very much
GIovanni