Fields found that when the bacteria accounted for 100 parts per million of the grain, the beetles' supercooling point ______.

A．They may cool the grain to - 10℃.
B．They can weaken the beetles' resistance to coldness.
C．They can be grown on most plant leaves.
D．They are used only in biological researches.

A．the bacteria are only effective against three species of beetles
B．Fields' technique is less effective in winter
C．Fields' technique may kill nearly all the beetles at -10℃
D．Insects may develop resistance against the bacteria