Section 17D. In any city in which the term of office of mayor is four years under the provisions of section seventeen C, upon the filing with the city clerk of a petition, which petition shall be subject to the provisions of section seven or section seven A of chapter fifty-three, signed by at least five per cent of the number of registered voters residing in said city at the last regular city election, the city clerk shall place upon the ballot for the next regular city election to be held not less than sixty days after the date of the filing of such petitions the following question:?
''Shall the term of office of mayor of the city of
___ be two years?''
If a majority of the votes cast in answer to said question is in the affirmative, the term of office of the mayor of said city shall thereafter be for two years and until the election and qualification of his successor, beginning with the next regular city election following the acceptance of this question.