Question: When did the city that is the twinned administrative body of Mainz (the start time of this statement is 1978) have a population of 791413?
Options:
2007-08-01
2016-06-30
1994-03-15
2018-01-01
2016-11-30
1981-03-31
2013-01-17
1959-04-06
2013-12-31
2015-07-09