9. The fact that he had been a monkey's uncle had been kept secret for nearly fifty years.

Ans.

9. For nearly fifty years the distant cousins had kept secret the fact that he had been a monkey's uncle.

Here the agent of action is again hidden. The "distant cousins" has been supplied as the agent here. The action that they are taking is keeping secret. Remember to use the "had kept secret" in order to preserve the past perfect tense.