A tube with both ends open floats vertically in water. Oil with a density 800 kg/`m^3` is poured into the tube. The tube is filled with oil upto the top end while in equilibrium. The portion out of the water is of length 10 cm. The length of oil in the tube is `10 alpha cm`. Find `alpha` (assume effect of surface tension is neglible):
A. 50 cm
B. 60 cm
C. 90 cm
D. 100 cm