As to a referendum of an enacted ordinance if, within the ten-day period mentioned in Sec. 5.03 (C), the petitioners' committee files with the City Clerk petitions signed by registered electors of the city consisting of at least 2.5% of the total number of qualified voters registered to vote at the last regular city election, then said ordinance shall not become effective until 45 days after its enactment. If a sufficient petition (Sec. 12.05 (A)) is filed within said 45 days, the ordinance shall remain suspended until the issue is determined.