An important factor that plays a part in susceptibility （影响） to colds is age．A study done by the University of Michigan School of Public Health indicated the fact that seems to hold true for the general population．Babies are the group suffering most from colds, averaging more than six colds in their first years．Boys have more colds than girls up to age three．After the age of three, girls are more susceptible than boys, and teenage girls average three colds a year to boy's two． The general frequency of colds continues to decline into adults．Elderly people who are in good health have as few as one or two colds annually．One exception is found among people in their twenties, especially women, who show a rise in cold infections（感染）, because people to this age group are most likely to have young children．Adults who delay having children until their thirties and forties experience the same sudden increase in cold infections． The study also found that economics （经济状况） plays an important role．As income increases, the frequency at which colds are reported in the family decreases．Families with the lowest income suffer about a third more colds than families at the upper end．Lower income generally forces people to live in closer quarters than those typically occupied by wealthier people, and crowding increases the opportunities for the cold virus to travel from person to person．Low income may also influence diet．The degree to which poor nutrition affects susceptibility to colds is not yet clearly proved, but an inadequate diet is suspected of lowering resistance generally． 68. What factors cause people to catch colds easily? A. Age and pregnancy．             B. Health and pregnancy． C. Health and economics．                  D. Age and economics． 69. Why does the lowest income lead to the rapid spread of colds? A. Because poor people can't afford to go to see a doctor． B. Because balanced diet has definitely affected poor people． C. Because people with the lowest income have limited living space． D. Because cold virus don't have opportunities to travel in poor families． 70. How does the study prove that age plays a part in susceptibility to colds? A. Using comparison．  B. Using graphs． C. Using description．  D. Using argument． 71. What can you learn from the passage? A. Families with the lowest income suffer less colds than rich families． B. The general frequency of colds are likely to increase among the poor． C. Women in their twenties are most likely to avoid cold infections． D. The general frequency of colds continue to rise into the adults．