Object Oriented Data Modeling
An abstract class is which of the following?

A class that has no direct instances, but whose descendants may have direct instances.
A class that has direct instances, but whose descendants may not have direct instances.
A class that has direct instances, but whose descendants may have direct instances.
A class that has no direct instances, but whose descendants may not have direct instances

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Object Oriented Data Modeling
Aggregation is which of the following?

Expresses an is-a relationship and is a stronger form of an association relationship.
Expresses a part-of relationship and is a stronger form of an association relationship.
Expresses an is-a relationship and is a weaker form of an association relationship.
Expresses a part-of relationship and is a weaker form of an association relationship.

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