Which of these phase comprises post-project support activities： activities that happen after a project is successfully implemented and handed off？ Select exactly 1 answer（s） from the following：（）。

A.plan
B.design
C.operate
D.implement

A.providing a step-by-step plan that details the installation and service-commission tasks required in order to create a controlled-implementation environment that emulates a customer network
B.assessing the ability of site facilities to accommodate proposed infrastructure changes
C.developing and executing proof-of-concept tests， validating high-level infrastructure design， and identifying any design enhancements
D.improving the network management system and the performance and functionality of infrastructure operations

A.plan
B.design
C.implement
D.prepare