California Code, Code of Civil Procedure - CCP § 2030.220

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

(b) If an interrogatory cannot be answered completely, it shall be answered to the extent possible.

(c) If the responding party does not have personal knowledge sufficient to respond fully to an interrogatory, that party shall so state, but shall make a reasonable and good faith effort to obtain the information by inquiry to other natural persons or organizations, except where the information is equally available to the propounding party.