The water level of oceans rises and falls alternately twice a day. This movement of water is called the tide. Tides are causer by the pull of the sun and the moon on the earth' s surface; since the moon is closer, it affects the tides more than the sun. When the moon is directly overhead, it actually pulls on the water that is below it. This causes the water level to rise because the water is pulled away from the earth. As the moon disappears over the horizon, the pull lessens and the water level settles back towards the ocean bottom. When the water reaches its highest level, we have high tide. And when the water comes to its lowest level, we have low tide from its lowest point, the water rises gradually for about six hours until it reaches high tide. Then it begins to fall continuously for about six hours until it reaches low tide. Then the cycle begins again. High tide occurs   _   .
every 12 hours every 6 hours every 24 hours every 18 hours