Which of the following statements regarding duration are true?

(I) Duration is a measure of interest-rate risk.

(II) If I buy $1000 worth of four different debt securities, the duration of the resulting portfolio is

the mean of the durations of those debt securities.

(III) All else equal, coupon bonds with shorter maturities have shorter durations.

A) I only

B) I, II, and III

C) I and III only

D) I and II only

E) II and III only