A game developer is writing a Python script to populate a $4\times 4$ 2D array representing a terrain elevation map. Each cell in the grid represents the height at a specific grid coordinate and is assigned a value returned by the function sample_elevation().

Complete the Python program in the code block below by filling in the five gaps on lines 9, 12, 14, and 16 so that the 2D array is correctly populated.

1  terrain_map = [ [0.0, 0.0, 0.0, 0.0],
2                  [0.0, 0.0, 0.0, 0.0],
3                  [0.0, 0.0, 0.0, 0.0],
4                  [0.0, 0.0, 0.0, 0.0] ]
5
6  y = 0
7  while y < 4:
8
9      x = _____
10     while x < 4:
11
12         terrain_map[ _____ ][ _____ ] = sample_elevation()
13
14         _____
15
16     _____