参考答案：C

解析：[听力原文]Most waves are caused by winds, though earthquakes on the ocean bed can also cause them, and tides are caused by the effect of the gravity of the moon on the earth and also, although to a lesser extent, the sun’s gravity. One bulge of water is drawn upon the side of the earth actually facing the moon; another bulge of water is drawn on the earth’s far side.

[分析]： 事实细节的找寻和判断。 题干问潮水形成的原因。根据原文tides are caused by the effect of the gravity of the moon on the earth and also... the sun’s gravity可知，C的说法“月亮和太阳的引力引起潮水”与原文相符，因此该项是正确答案。[解题关键] 1)根据题干中关键词判断听的重点。本题中的关键词是tides和caused by，由此可迅速排除第一句和最后一句；2)绝对化的表达一般是错误的，如A和B中 alone的使用，使全句过于绝对化，因此是错误的。