The ______ of California grey whale is about sixty years.

听力原文： A four-year study conducted by the Infant Testing Center in San Francisco, California, suggests that babies feel more comfortable around other babies than with strange adults. According to the study, babies benefit by being with their fellow infants daily. Whereas a baby might show fear of an adult stranger, he is likely to smile and reach out to an unfamiliar infant. By the time babies are one year old, they have begun to form. friendships of a sort.

The above findings, based on the observation of 100 babies aged three months to three years, might prove interesting to working parents who must find day-care for their babies. Family care in a private home, with several babies together, is probably the ideal way to cam for babies under three. Dr. Benjamin Spock, a well-known pediatrician and an author of books about babies, supports the idea. He says that family day-cam is sounder in theory than hiring a housekeeper or a babysitter.

(30)

A．Strange adults.

B．Familiar adults.

C．A baby sitter.

D．Fellow infants.