which required option, could be downloaded from Cisco.com?

While using 5.x signatures to enable Cisco IOS IPS, which required option, could be downloaded
from Cisco.com?

A.
Built-in signatures

B.
public key

C.
SDF files (128MB.sdf, 256MB.sdf, attack.drop.sdf)

D.
Signature Micro-Engines and IME