Men could directly turn back to Greece and Rome for instruction, for correction, and for inspiration in the following fields of their work except ______.

A．plants that infest human habitations
B．the fact that bacteria are really vegetables
C．the growing stage of the bacteria as opposed to the dormant stage
D．the difficulty in classifying the . types of bacteria

A．heat wave
B．time of high humidity
C．windless period
D．shortage of fuel oil