I really, really want to give a full answer, but because this is homework, I'll try to nudge you there.
The cause of this phenomenon is the tilt of the Earth's axis. As the Earth revolves around the Sun, the tilt changes, the seasons progress, and more or less sunlight hits a given region of Earth, depending on where it is and which way the axis is tilted. This tilt means that the Sun appears to be at a different angle in the sky, which in turn means that in winter, the Sun appears lower in the sky. In summer, the Sun appears higher in the sky. So as September progresses, the Sun should start getting lower and lower. ?This graphic sums it up:
Fill in the rest and you'll get the answer
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