Children who are brought up by two parents grow up to be cleverer than those raised by just one person, new research suggests. Being with both parents in the earliest years of life leads to a child developing more brain cells, the scientists believe. Being brought up by both parents causes boys to have better memory and learning functions. By contrast, it causes girls to develop improved co-ordination and sociability. It is believed that babies with two parents tend to get more attention and more stability , and that they are less likely to suffer emotional distress  in the first years of life. This leads to greater brain cell production ------ for boys it is grey matter brain cells that develop and for girls is white matter brain cells. The researchers from Canada studied mice and experimented by creating one-parent and two-parent family groups. They then measured the offspring's  brain cell development from birth to adulthood. Adult mice with the highest number of brain cells turned out to be those who had been brought up by two parents rather than one. As babies they had received more attention and more nursing as both parents took turns to lick and tend to their youngsters, said Dr Samuel Weiss. As a result, the babies with two parents are less likely to suffer early life hurt which can have a massive impact on how their brains develop in later life, the research shows. However, what did surprise the researchers is that female babies who grew up with both parents turned out to be good single mothers, as if good parenting was passed on. The researchers said," In the mouse model, parenting and the environment directly _ adult brain cell production. It is possible that similar effects could be seen in other mammals, such as humans." The researchers proved their idea   _  .
by questionnaire on parents. by experiment on mice. by raising babies themselves. by experiment on humans.
Question: Children who are brought up by two parents grow up to be cleverer than those raised by just one person, new research suggests. Being with both parents in the earliest years of life leads to a child developing more brain cells, the scientists believe. Being brought up by both parents causes boys to have better memory and learning functions. By contrast, it causes girls to develop improved co-ordination and sociability. It is believed that babies with two parents tend to get more attention and more stability , and that they are less likely to suffer emotional distress  in the first years of life. This leads to greater brain cell production ------ for boys it is grey matter brain cells that develop and for girls is white matter brain cells. The researchers from Canada studied mice and experimented by creating one-parent and two-parent family groups. They then measured the offspring's  brain cell development from birth to adulthood. Adult mice with the highest number of brain cells turned out to be those who had been brought up by two parents rather than one. As babies they had received more attention and more nursing as both parents took turns to lick and tend to their youngsters, said Dr Samuel Weiss. As a result, the babies with two parents are less likely to suffer early life hurt which can have a massive impact on how their brains develop in later life, the research shows. However, what did surprise the researchers is that female babies who grew up with both parents turned out to be good single mothers, as if good parenting was passed on. The researchers said," In the mouse model, parenting and the environment directly _ adult brain cell production. It is possible that similar effects could be seen in other mammals, such as humans." The researchers proved their idea   _  .
A. by questionnaire on parents.
B. by experiment on mice.
C. by raising babies themselves.
D. by experiment on humans.
Answer: