A 1, A2, ..., A 100 are arranged in order of size.

When A 1, A2, ..., A 100 is arithmetic progression and the first term is tolerance, there are the most elements in set B.

Choose two numbers from 100, and the difference must also belong to A 1, A2, ..., A 100.

The maximum number of elements in set b = c (100,2) =100 * 99/2 = 4950.