The math problem seems very easy, but in fact it is very difficult_______.

A．to work out

B．being worked out

C．working out

D．to be worked out.

根据《国家电网公司信息安全加固实施指南（试行）》，信息安全加固流程中，下列（）不属于验证总结阶段的工作内容。

A.系统核查

B.加固验证

C.数据整理

D.数据备份