Why Are Treasury Bills Among the Safest Investments in the World

Why are Treasury bills among the safest investments in the world?

A) They are short-term investments and therefore extremely sensitive to interest rate changes.
B) They are long-term investments and therefore extremely insensitive to interest rate changes.
C) They are short-term investments and therefore fairly insensitive to interest rate changes.
D) They are backed by the full faith and credit of the U.S.government.
E) Both (c) and (d) .