CAL. CCP. CODE § 2033.220 : California Code - Section 2033.220

(a) Each answer in a response to requests for admission shall be as complete and straightforward as the information reasonably available to the responding party permits.

(b) Each answer shall:

(1) Admit so much of the matter involved in the request as is true, either as expressed in the request itself or as reasonably and clearly qualified by the responding party.

(2) Deny so much of the matter involved in the request as is untrue.

(3) Specify so much of the matter involved in the request as to the truth of which the responding party lacks sufficient information or knowledge.

(c) If a responding party gives lack of information or knowledge as a reason for a failure to admit all or part of a request for admission, that party shall state in the answer that a reasonable inquiry concerning the matter in the particular request has been made, and that the information known or readily obtainable is insufficient to enable that party to admit the matter.