21. The average value of $\frac { 1 } { x }$ on the closed interval $[ 1,3 ]$ is\\ (A) $\frac { 1 } { 2 }$\\ (B) $\frac { 2 } { 3 }$\\ (C) $\frac { \ln 2 } { 2 }$\\ (D) $\frac { \ln 3 } { 2 }$\\ (E) $\ln 3$\\