As can be seen from the time schedule of "Earth Hour" operation, the last Saturday in March is around the vernal equinox every year. At this time, the world is divided equally between day and night, and the sunset time is 18: 00, and 20:30-2 1:30. It is night all over the world. Option C is the third Saturday in September, that is, around the autumnal equinox, which also meets the above requirements. Therefore, every year,

So choose: C.