单项选择题

Human babies are born in immature, helpless states, owing to ______.

A．the early birth of human fetuses
B．the big brains of the fetus
C．the constricted birth canals of the mother
D．both big brains and constricted birth canals

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单项选择题

Judging from the context, the word forage (Line 6, Paragraph 6) most probably means ______.

A．watch out in case some danger occurs
B．look after sb, especially the young baby
C．find out sth. perilous
D．wander in search of food or provisions

单项选择题

This evolution of human brains was not without its difficulties for mothers, because ______.

A．human brains started to grow larger
B．our ancestors walked upright on two legs
C．it altered our ancestors' skeletons
D．it makes the passage of big-brained infants difficult