GWD 19-26 请教

Q26:

In his experiments with gravity, Isaac Newton showed how the motion of each planet in the solar system results from the combined gravitational pull of the Sun and of all the other planets, each contributing according to their mass and distance from the others.

1. of all the other planets, each contributing according to their

2. of all the other planets, with each of them contributing according to their

3. all the other planets, each of which contributing according to its

4. all the other planets, each contributing according to its

5. all the other planets, each of which contribute according to their

答案是D, 这里的each 是作什么词性呢,后面要用its

代other planets

如果撇开这道题 那么each of ...中的each 是什么磁心呢

谢谢