People born in autumn live longer than those born in spring and are less likely to fall chronically ill when they are older, according to an Austrian scientist, Using census (人口普查) data for more than one million people in Austrian. Denmark and Australia, scientists at the Max Plank Institute for Demographic (人口统计的) Research found the month of birth was related to life expectancy over the age of 50. Seasonal differences in what mothers ate during pregnancy, and infections occurring at different times of the year could both have an impact on the health of a new - born baby and could influence its life expectancy in old age. "A mother giving birth in spring spends the last stage of her pregnancy in winter, when she will eat fewer vitamins than in summer. " said one of the scientists. "When she stops breast - feeding and starts giving her baby normal food, it’s in the hot weeks of summer when babies are inclined to infections of the digestive system. " In Austria, adults born in autumn lived about seven months longer than those born in spring, and in Denmark adults with birthdays in autumn outlived those born in spring by about four months. In the southern hemisphere, the picture is similar. Adults born in the Australian autumn lived about four months longer than those born in the Australian spring. The study focused on people born at the beginning of the 20th century, using death certificates and census data. Although nutrition at all times of the year has improved since then, the seasonal pattern persists.
The passage is mainly about（）。
A. why people born in autumn are free from chronic illness
B. how the seasonal pattern affects the health of new - born babies
C. how the month of birth is related to life expectancy
D. how nutrition in different seasons influences life expectancy