In relation to a Graph Database which of the following are correct?

Question $5$Select one or more:

a$.$

In a graph database we represent data in terms of graphs having two fundamental units being nodes and relationships and while noes may have properties relationships are not allowed to have properties.

b$.$

Neo$4$j does not support the join operation.

c$.$

Graph databases can navigate through graphs to answer queries, and thus they always provide a better performance for queries than relational databases.

d$.$

Compared with key$-$value data stores, graph databases generally have more complex data models but are less scalable.