For simplicity, assume there are no duplicates and consider an array of size 3:
A[0] = v1
A[1] = v2
A[2] = v3
where the three values v1, v2, v3 are the values 1, 2, 3 in some order
Bubble sort must sort them so that the result is
A[0] = 1
A[1] = 2
A[2] = 3
for each possible ordering v1, v2, v3 of 1, 2, 3.
Which of the values {v1, v2, v3} are compared by the first comparison of Bubble.sort?