What method can be used to reduce the risk of overflowing the the primary RR BGP routing table?

In the multi-level RR scenario, a large number of secondary RRs will be connected to the primary RR, which may cause the BGP routing table of the primary RR to overflow.

What method can be used to reduce the risk of overflowing the the primary RR BGP routing table?
A . ORF
B . Routing Control
C . Route Aggregation
D . Set the same Cluster-ID

Answer: C