A cylindrical pail containing water drains into a cylindrical tub 40\:cm across and 50\:cm deep, while resting at an angle of 45^\circ to the horizontal, as shown. How deep is the water in the tub when its level reaches the pail?
Answer Choices
A. 10\:cm
B. 20\:cm
C. 30\:cm
D. 35\:cm
E. 40\:cm