California Code of Civil Procedure
Code of Civil Procedure section 2030.010
(a) Any party may obtain discovery within the scope delimited by Chapter 2 (commencing with Section 2017.010 ), and subject to the restrictions set forth in Chapter 5 (commencing with Section 2019.010 ), by propounding to any other party to the action written interrogatories to be answered under oath.
(b) An interrogatory may relate to whether another party is making a certain contention, or to the facts, witnesses, and writings on which a contention is based. An interrogatory is not objectionable because an answer to it involves an opinion or contention that relates to fact or the application of law to fact, or would be based on information obtained or legal theories developed in anticipation of litigation or in preparation for trial.