Consider the following relation and query:

B(bid, accNo), accNo is a key.

A(accNo, code, balance), accNo is a key.

Select B.bid, Sum(A.balance)

From B, A

Where B.accNo=A.accNo

Group by B.bid

This query computes the sum of the balances at each branch. Assume that there a clustered B+ tree index on B.bid and a hash index on A.accNo. Discuss the plans of evaluating the query with lowest I/O cost. If A and B have |A| and |B| pages, and each page has 100 records, what is the I/O cost? (Hint: use index to reduce the cost of join and sorting cost for aggregation).