Thank you, Bitbucket. This question is very nice.
Initially I picked 6 and then changed to 5, thinking of the S in A has 6 unique value and then R in A has only 5 unique values and was expecting it will work like LEFT JOIN and was wrong.
Also I was expecting the data will be dirty, but seeing the dataset each row has its unique representation with the combination of A-R and B-R.
The first and the hardest SQL statement I have wrote- "select * from customers" - and I was happy and felt smart.