Spark plugs cause the fuel mixture to()

A. their brains could not function efficiently any more
B. the full development of both sides of their brain was achieved
C. their bodies stopped working
D. they were very old

A. People are more likely to catch cold in the winter.
B. Colds are so common that even the strongest person cannot avoid it.
C. Old people are likely to catch cold because they are unpleasant.
D. Colds can never be cured without the help of medicine.