单项选择题

(D)

Millions of stars are travelling about in space. A few form groups which journey together, but most of them travel alone. And they travel through a universe so large that one star seldom comes near to another. For the most part each star makes its journey in complete loneliness, like a ship on an empty ocean. The ship will be well over a million miles from its nearest neighbor. From this it is easy to understand why a star seldom finds another anywhere near it. We believe, however, that some two thousand million years ago, another star wandering (漫游) through space, happened to come near our sun. Just as the sun and the moon raise tides (潮汐) on the earth, so this star must have raised tides on the surface of the sun. But they were very. different from the small tides that are raised in our oceans; a large tidal wave must have travelled over the surface of the sun, at last forming a mountain so high that we cannot imagine it. As the cause of the disturbance (动荡) came nearer, so the mountain rose higher and higher. And before the star began to move away again, its tidal pull had become so powerful that this mountain was tom to pieces and threw off small parts of itself into space. These small pieces have been going round the sun ever since. They are the planets (行星).

The expression "the cause of the disturbance" refers to______.