GWD-29-Q26 ---D项加上of对么

Of course,it is better to add an "of" at the begining of the choice(D).
It is the combined gravitational pull of and (the combined gravitational pull of) the others that results in the motion of each plannet,keeping them moving in balance.Adding an "of" can clear the ambiguity while omiting does not make a gramar mistake.It a trap here since A sounds better if "their" changes to "its",which reffers to a single "each".

BTW,It is a use of idiom here:each contributing ....
Remember the grossay of such use:
1.n + a. :I am jtrchina,a CDer famous for his kindness.
2.n + preposition :I am jtrchina,a member of CD.
3.n + doing :I am jtrchina,a member writing posts.
4.n + ~ed: I am jtrchina,a member thought as friendly.
5.n + to do:I am jtrchina,a friend to help you overcome the gmat.

Hope it helps!