Q: In this task, you need to answer the given multiple-choice question on geometry. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.
Problem: if the sides of a triangle are 4 cm , 9 cm and 13 cm , what is its area ? 
Options: a ) 18 , b ) 20 , c ) 22 , d ) 26 , e ) 28
A:
a