Which two are valid choices for digital certificates when setting up two-way SSL between Salesforce and an external system. Choose 2 answers

Which two are valid choices for digital certificates when setting up two-way SSL between Salesforce and an external system. Choose 2 answers
A . Use a trusted CA-signed certificate for salesforce and a trusted CA-signed cert for the external system
B. Use a trusted CA-signed certificate for salesforce and a self-signed cert for the external system
C. Use a self-signed certificate for salesforce and a self-signed cert for the external system
D. Use a self-signed certificate for salesforce and a trusted CA-signed cert for the external system

Answer: C,D