A、They are a reasonable explanation for the observed birth-order differences. B、They are incomplete in explaining the observed b

A recent report says eldest siblings score higher in IQ tests than younger brothers and sisters because they get more mental stimulation from parents.
   They may be jokingly referred to as PFBs—precious first-borns—on popular parenting websites, but a study says first-born children really do reap the benefits of being number one.
   Research by the University of Edinburgh has found that first-born children have superior thinking skills to their younger siblings because they get more mental stimulation from their parents.
   While the study has found that parents give all their children the same levels of emotional support, the firstborn generally received more help with tasks that develop thinking skills.
   The study, published in the Journal of Human Resources, observed nearly 5,000 children from pre-birth to age 14, with children assessed every two years.
   Researchers said the findings went some way to explaining the so-called birth-order effect—which means children born earlier in a family have better wages and higher levels of education later in life.
   First-born children scored higher on tests including reading, matching letters, names, reading single words aloud and picture vocabulary tests.
   Researchers also gathered information on environmental factors such as family background and economic conditions.
   The study found parents changed their behaviour as they had more children, giving less mental stimulation and taking part in fewer activities like reading with the child, crafts and playing musical instruments. Mothers also took part in more risky behaviours such as smoking during pregnancy with subsequent children.
   Dr. Ana Nuevo-Chiquero of Edinburgh University’s school of economics said, " Our results suggest that broad shifts in parental behaviour are a plausible explanation for the observed birth-order differences in education and labor market outcomes. "
Questions 12 to 15 are based on the passage you have just heard.
12. Why do eldest siblings generally score higher in IQ tests than their younger brothers and sisters?
13. What has the research by the University of Edinburgh found?
14. In which of the following tests do first-born children score higher than their younger siblings?
15. What does Dr. Ana Nuevo-Chiquero say about broad shifts in parental behaviour?