Millions of stars are traveling 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.

We believe, however, that some two thousand million years ago, another star wandering through space, happened to come near our sun. Just as the sun and the moon raise tides 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 torn 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).

Millions of stars are______.

A．following a regular path in space

B．always travelling together

C．seldom wandering about in the universe

D．moving about without a fixed course