21. A U-shaped tube contains mercury (density 13.6 g/cm³ ). If a 13.6 cm column of water is poured into the left arm, what is the difference in height between the mercury levels in the two arms? (Assume the system reaches equilibrium)
(A) The mercury level in the left arm is 12.6 cm higher.
(B) The mercury level in the right arm is 1 cm higher.
(C) The mercury level in the left arm is 1 cm higher.
(D) The mercury levels in both arms are at the same height.