Congressional committee assignments are one way that political parties demonstrate control over

The correct answer for the question that is being presented above is this one: "b. party members in Congress." Congressional committee assignments are one way that political parties demonstrate control over b. party members in Congress 

Here are the following choices:
a. the party-in-the-electorate 
b. party members in Congress 
c. interest groups 
d. all of the above

Could you give me some more details so i can you?