HBase one to many 'relationship' storage

I'm pondering the best way to implement a one to many relationship in HBase.

Say an ACCOUNT has many TRANSACTION(s). Is it better to

a) Add columns to a transactions: column family on the ACCOUNT table, i.e. transactions:1:amount, transactions:2:amount

b) Only store the key(s) of each TRANSACTION relating to an account in in the transactions: column family of ACCOUNT, and do a lookup of each transaction found on a separate TRANSACTION table?