<image>The flag of Kangaria is a rectangle with side-lengths in the ratio $3: 5$. The flag is divided into four rectangles of equal area as shown. What is the ratio of the length of the shorter sides of the white rectangle to the length of its longer sides?
A. $1: 3$
B. $1: 4$
C. $2: 7$
D. $3: 10$
E. $4: 15$