The shorter leg of a \( 30 ^ { \circ } - 60 ^ { \circ } - 90 ^ { \circ } \) triangle measures \( 4 \sqrt { 2 } \) kilometers. What is the measure of its hypotenuse?
A. \( 8 km \) .
B. \( 8 \sqrt { 2 } km \) .
C. \( 8 \sqrt { 3 } km \) .
D. \( 4 \sqrt { 6 } km \) .