California Code, Code of Civil Procedure - CCP § 2030.410

At the trial or any other hearing in the action, so far as admissible under the rules of evidence, the propounding party or any party other than the responding party may use any answer or part of an answer to an interrogatory only against the responding party.  It is not ground for objection to the use of an answer to an interrogatory that the responding party is available to testify, has testified, or will testify at the trial or other hearing.