Why is cloture very difficult to achieve?

a. senators are famous for their debating prowess and would not easily give up to cloture.
b. the two-thirds vote of the senate is almost impossible to attain.
c. cloture can only be achieved by a three-fifths vote, and parties don't usually have that kind of majority.
d. even the threat of cloture will cause senators to get votes to stop it.

The good answer is:

c. Cloture can only be achieved by a three-fifths vote, and parties don't usually have that kind of majority.

Explanation:

Cloture is the only legal resource to overcome filibuster. Cloture is a limit on the time allowed for consideration of a bill. But to get it approved, 60 votes in the Senate, or three-fifths, are neccessary. No party usually enjoys the privilege of being able to gather such a majority.

the answer is:

c. economic output