Why Are So Many Senators Reluctant to Use Cloture

Why are so many senators reluctant to use cloture?

A) For fear of setting a precedent to be used against them when they want to filibuster
B) Because it has always been used as a stepping stone to the Speaker's job,and once used,they may not get a second chance at the job
C) Because,in most cases,the House or Senate usually considers only the bills that obtain a favorable committee report
D) Because they have to stay in their positions to monitor the bureaucracy,otherwise there would be no congressional oversight
E) Because the size and complexity of today's government make it impossible for any one person to understand all the issues before the Senate