Exadata grid disks; disk group and redundancy (external redundancy)

 I have a doubt about an certification question... some one can help-me?

The books say: 

Book: "Oracle Exadata Recipes" 

Pg. 59 - "When we created these ASM disk groups, we specified normal redundancy. With Exadata, external redundancy is not an option - you either need to use normal or high redundancy. With normal redundancy, each extent is mirrored to a different cell, and with high redundancy, they are mirrored via ASM to two additional cells. Specifically, extents are mirrored to partner disks in different failure groups."

Pg. 120 - "On Exadata, the Oracle ASM external redundancy is not supported. On non-Exadata storage environments, many clients elect external redundancy and rely on host-based or array-based RAID to provide data protection. With Exadata the storage servers do not provide RAID protection; data is protected solely with Oracle ASM redundancy. You can elect to configure ASM disk groups with no redundancy, but this is not recommended for obvious reasons."