21. During the lag phase

A. cells just start growing
B. cells synthesize new proteins and membrane components
C. cells produce antibiotics to kill competing organisms
D. cells simply grow slowly

22. Concentration of the growth limiting substrate means

A. trace elements concentration
B. the concentration of the substrate that controls the growth rate of the cells
C. sugar concentration in the fermentation medium
D. dissolved oxygen concentration

23. Tower fermentors are used for

A. continuous penicillin production
B. continuous beer production
C. production of enzymes
D. batch production of beer

24. In wastewater treatment, which bioreactor is generally used?

A. Plug flow reactor
B. Trickle bed reactor
C. Tower fermentor
D. CSTR

25. The contents in the stirred tank reactor are mixed through

A. compressed air
B. a mechanical agitator
C. external pumping
D. any of these