The average concentration of oxygen in the boundary layers surrounding the bubbles $(C{o}^{**})$ in a reactor is normally determined by

O a measuring the steady state concentration of oxygen in the bulk liquid after inoculation of the reactor

Ob$.$ ultra$-$small dissolved oxygen probes

O c$.$ measuring the steady state concentration of oxygen in the bulk liquid prior to inoculation of the reactor

d$.$ laser based photographic

systems