The Main Reason for the Length of the Term of Office

The main reason for the length of the term of office on the Federal Reserve Board is that

A) this time period corresponds closely to the lag time between actions by the Fed and their impact on the economy.
B) this term gives the individual governors the same job security as their peers in private commercial banks.
C) many years of in-service education are required before board members can make substantive contributions.
D) given the stringent requirements for membership, it has proven difficult to fill positions any more frequently.
E) long appointments limit opportunities for political influence over the board.