Answer:
Option C. 
Step-by-step explanation:
step 1
In the right triangle ABD find the length side AB
Applying the Pythagoras Theorem



step 2
In the right triangle ABD
we know that

-------> equation A
step 3
In the right triangle ABC
------> equation B
Remember that

so equate equation A and equation B solve for DC






step 4
In the right triangle BDC find the length side BC
Applying the Pythagoras Theorem


