The Swiss Parliament consists of two houses: the Council of States which has 46 representatives (two from each canton and one from each half-canton) who are elected under a system determined by each canton, and the National Council, which consists of 200 members who are elected under a system of proportional representation, depending on the population of each canton. Members of both houses serve for 4 years. When both houses are in joint session, they are known collectively as the Federal Assembly. Through <answer>referendums</answer>, citizens may challenge any law passed by parliament and through initiatives, introduce amendments to the federal constitution, thus making Switzerland a direct democracy.
What can citizens use to challenge any law passed by Parliament?
referendums