Motherhood may make women smarter and may

题目：

Motherhood may make women smarter and may help prevent dementia in old age by bathing the brain in protective hormones, US reseachers reported on Thursday.
Tests on rats show that those who raise two or more litters of pups do considerably better in tests of memory and skills than rats who have no babies, and their brains show changes that suggest they may be protected against diseases such as Alzheimer’s. University of Richmond psychology professor Craig Kinsley believes his findings will translate into humans.
"Our research shows that the hormones of pregnancy are protecting the brain, including estrogen, which we know has many neuroprotective effects, " Kinsley said.
"It’s rat data but humans are mammals just like these animals are mammals, " he added in a telephone interview. "They go through pregnancy and hormonal changes. "
Kinsley said he hoped public health officials and researchers will look to see if having had children protects a woman from Alzheimer’s and other forms of age-related brain decline.
"When people think about pregnancy, they think about what happens to babies and the mother from the neck down, " said Kinsley, who presented his findings to the annual meeting of the Society of Neuroscience in Orlando, Florida.
"They do not realize that hormones are washing on the brain. If you look at female animals who have never gone through pregnancy, they act differently toward young. But if she goes through pregnancy, she will sacrifice her life for her infant—that is a great change in her behavior that showed in genetic alterations to the brain. \

How do scientists know "motherhood may make women smarter"

A. Some researchers have told them.
B. Many women say so.
C. They know it by experimenting on rats.
D. They know it through their own experience.