(a) SENATORS. Senators shall be elected for terms of four years, those from odd-numbered districts in the years the numbers of which are multiples of four and those from even-numbered districts in even-numbered years the numbers of which are not multiples of four; except, at the election next following a reapportionment, some senators shall be elected for terms of two years when necessary to maintain staggered terms.

  (b) REPRESENTATIVES. Members of the house of representatives shall be elected for terms of two years in each even-numbered year.

  (c) QUALIFICATIONS. Each legislator shall be at least twenty-one years of age, an elector and resident of the district from which elected and shall have resided in the state for a period of two years prior to election.

  (d) ASSUMING OFFICE; VACANCIES. Members of the legislature shall take office upon election. Vacancies in legislative office shall be filled only by election as provided by law.