单项选择题

The sun shines because it is made of glowing gases. It gives off its own light. The moon shines too, but it doesn’t give off any light of its own. We can see the moon only because it reflects light from the sun. The moon travels around the earth about every twenty-nine days. During this time, the moon looks as if it changes size and shape. It seems to grow from a small part of a shining circle into a full circle. Then it becomes small again. The moon doesn’t really change. It is just because we see different parts of it lit up by the sun.

The moon seems to change size and shape because ______.