Which of the Following Experiments Would Help to Identify a Keystone

Which of the following experiments would help to identify a keystone species?

A) Introduce more of the suspected species into the area and observe the effects on other community members.
B) Introduce an invasive species related to the suspected species into the area and observe the effects on the suspected species.
C) Remove the suspected species from the area and observe the effects on other community members.
D) Introduce additional individuals of the suspected species' food source into the area and observe the effects on community members.