In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.

[EX Q]: rCFrfkmXbSHK, mztfkmXwez
[EX A]: fkmX

[EX Q]: UiaECWZf, gRyaECWLGRl
[EX A]: aECW

[EX Q]: ChKUlYUTQQBjZMZkgEed, lmQQBjZMZkgdEf
[EX A]:
QQBjZMZkg