What rule determines which members of congress will serve as chairs and which as members of each committee?

The correct answer to this open question is the following.

The rule that determines the members of Congress that will serve as chairs and which as members of each committee is the Seniority Rule.

Seniors have privileges everywhere and Congress is not the exception.  

In US politics, the Seniority Rule or Seniority system refers to the practice in Congress that gives special treatment and privileges to senior members of the Senate or the House of Representants. This means the congressmen that have served the country in Congress for a long time. These members -through the years- have become very powerful men and women that influence legislation, pick their offices in Congress and their assignments in the different committees.

this headline suggests that midterm congressional   elections are at the corner and trump administration wants to activate his base by vowing to clamp down on immigrants and immigration as trump had said during his election campaign around two years ago. when a baby is born in usa, the baby becomes citizen of usa for 85 years and trump wants to end this rule of birthright citizenship to control illegal immigration and this what he said in an interview""we’re the only country in the world where a person comes in and has a baby, and the baby is essentially a citizen of the united states for 85 years, with all of those benefits,” "