Select all statements that correctly describe distributed database management systems (DDBMSs).
a.When compared to a centralized DBMS, a distributed DBMS is usually more complex.
b.When a single database management system runs across multiple processors and disks, it is a DDBMS.
c.When compared to a centralized DBMS, a distributed DBMS usually has higher availability to the database clients.
d.If a database instance can be accessed by a database client over a computer network, that instance's underlying database management system is always a DDBMS.
e.If two or more database instances maintain the data for a single logical database schema, the underlying database management system is always a DDBMS.