California Code, Code of Civil Procedure - CCP § 2030.080

(a) The party propounding interrogatories shall serve a copy of them on the party to whom the interrogatories are directed.

(b) The propounding party shall also serve a copy of the interrogatories on all other parties who have appeared in the action. On motion, with or without notice, the court may relieve the party from this requirement on its determination that service on all other parties would be unduly expensive or burdensome.