RPMI and DMEM, supplemented with serum, antibiotics, glutamine and phenol red are routinely used for tissue culture of human cells in CO₂ incubators. In addition, sodium bicarbonate (NaHCO₃) and HEPES are used as buffering agents. The following statements were made about the media.

A. While 5% CO₂ is optimal for cells cultured in RPMI, the optimal CO₂ concentration for DMEM is 7.5-10%.

B. HEPES is necessary if cells are to be kept outside the incubator in room air for long periods.

C. NaHCO₃ is necessary if cells are to be kept outside the incubator in room air for long periods.

D. When cells grow rapidly in the culture medium for a few days, phenol red will turn the medium pink/red.

Which one of following options represents the correct combination of all the statements?