If a DBMS enforces an UPDATE RESTRICT option on the referential integrity constraint between CLIENT and AGENT in the HAPPY INSURANCE database, what will be the outcome after a user tries to change the AgentID value in the last record to A5?

a. Value A4 appears once in column AgentID in AGENT, and value A4 appears once in column ClientAgent in table CLIENT
b. Value A5 appears once in column AgentID in AGENT, and value A5 appears once in column ClientAgent in table CLIENT
c. Value A5 appears once in column AgentID in AGENT, and value A5 does not appear in column ClientAgent in table CLIENT
d. Value A5 does not appear in column AgentID in AGENT, and value A5 appears once in column ClientAgent in table CLIENT