Available choices:
1). magma;
2). plates;
3). two;
4). algae;
5). sun;
6). earth;
7). moon;
8). spring;
Q: You are presented with the quiz "what forms tides?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "the gravitational pull of the Moon on Earth 's oceans causes the tides" and "gravity causes tides". So, what's the best answer to the question?
A:
7).