For the ZYX Company, the ER Diagram is shown below. Review it and answer the question below.

EMPLOYEE entity is related to the CUSTOMER entity using relationships 'Serves' and 'SignsUp'. For both relationships, the cardinality on the EMPLOYEE entity side is $||.$ For the 'Serves' relationship, the cardinality on the CUSTOMER entity side is $0|.$ For the 'SignsUp' relationship, the cardinality on the CUSTOMER entity side is $0<.$ EMPLOYEE entity has three attributes: EmpID $($unique$),$ OfficePhoneNumber $($optional$),$ and CellPhoneNumber $($multiple$).$ CUSTOMER entity has three attributes: CustID $($unique$),$ CustAge $($derived$),$ and CustBDate. CUSTOMER entity is related to itself using the relationship 'Refers'. On one side of the 'Refers' relationship, the cardinality is $0|,$ while on the other side, the cardinality is o$<.$

Which of the following is true according to the ZYX Company ER diagram?

We cannot keep track of employees that sign up multiple customers

We cannot keep track of employees that do not serve any customers

We cannot keep track of employees that serve multiple customers

We cannot keep track of employees that do not sign up any customers