3. Four identical small rectangles are put together to form a large rectangle as shown. The length of a shorter side of each small rectangle is $10 \mathrm{~cm}$. What is the length of a longer side of the large rectangle?
A $50 \mathrm{~cm}$
B $40 \mathrm{~cm}$
C $30 \mathrm{~cm}$
D $20 \mathrm{~cm}$
E $10 \mathrm{~cm}$