A、Work. B、Food. C、Social status. D、Age. D根据“A critical factor that plays a part in people’s susceptibility to colds is age.”，可以选

A critical factor that plays a part in people’s susceptibility to colds is age. The University of the Michigan School of Public Health has done a study that revealed some general rules for the public. It says that infants are the most cold-ridden group. On an average, infants will catch more than six colds during their early years. Besides, boys have more colds than girls when they are under the age of three. When they grow older than three years old, it is easier for girls to catch cold than the boys. Girls might catch three colds, while boys catch two on average.
    Generally speaking, the incidents of colds continue to decline while people are growing older. Elderly people who are in good health have as few as one or two colds every year. One exception is found among people in their twenties, especially women, who show a rise in cold infections. This is because people in this age group are most likely to have young babies. Adults who delay having children until their 30s and 40s experience the same sudden increase in cold infections.
    The study also found that economics 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 more limited space than those occupied by rich people, and crowding will increase the possibility of cold infections.