Which of the following is correct?

a. The fact that most NoSQL databases adopt an eventual consistency approach is due to the CAP theorem, which states that strong consistency cannot be obtained when availability and partitioning have to be ensured.

b. Replicas in a distributed NoSQL environment relate to making periodic backups of the database to a second system.

c. Stabilization relates to the waiting time between the start-up of a NoSQL system and when the system becomes available to receive user queries.

d. Some relational constructs, such as the many-to-many relationship, are harder to express using graph databases.