Which statement is true, as relates to classful or classless routing?

A.
Classful routing protocols send the subnet mask in routing updates.

B.
RIPv1 and OSPF are classless routing protocols.

C.
Automatic summarization at classful boundaries can cause problems on discontiguous subnets.

D.
EIGRP and OSPF are classful routing protocols and summarize routes by default.

Explanation:
http://www.ciscopress.com/articles/article.asp?p=174107&seqNum=3
RIPv1, RIPv2, IGRP, and EIGRP all auto-summarize classful boundaries by default (OSPF does not).To
make discontiguous networks work, meaning you don’t want classful boundries to summarize, you
need to turn off auto-summary.