A cylindrical tube open at both ends has a fundamental frequency $\mathrm{f}$ in air. The tube is dipped vertically in water so that half of it is in water. The fundamental frequency of the air column is now........
(A) $\mathrm{f} / 2$
(B) $\mathrm{f}$
(C) $3 \mathrm{f} / 4$
(D) $2 \mathrm{f}$