A．Age．
B．Sex．
C．Nutrition．
D．Income．

A．Because they are busy with their career in this period．
B．Because they aren't conscious of health．
C．Because people of this age belong to the most cold-ridden group．
D．Because having children makes them easy to be infected．

A．The lower-income people are very sensitive to cold bacteria．
B．The lower-income people have lower resistance．
C．The crowded space and poor diet make the lower-income group easy to be infected．
D．The bad mood of the lower-income group makes them very easy to be infected．