[单项选择]胎儿头部不容易通过母畜产道的原因是（）
A. 头部骨化较完全，无伸缩
B. 母畜骨盆狭窄
C. 母畜骨盆畸形
D. 子宫颈开张不全
E. 荐坐韧带未松弛

{{B}} Questions 47 to 56 are based on the following passage.{{/B}}
The way in which people use social space reflects their social relationships and their ethnic identity. Early immigrants to America from Europe brought with them a {{U}} (47) {{/U}} style of living, which they retained until late in the 18th century. Historical records document a group-Oriented {{U}} (48) {{/U}} , in which one room was used for eating, entertaining guests, and sleeping. People ate soups from a communal pot, {{U}} (49) {{/U}} drinking cups, and used a common pit toilet. With the development of ideas about individualism, people soon began to shift to the use of {{U}} (50) {{/U}} cups and plates; the eating of meals that included meat, bread, and vegetables {{U}} (51) {{/U}} on separate plates; and the use of private toilets. They began to build their houses with separate rooms to {{U}} (52) {{/U}} guests — living rooms, separate bedrooms for sleeping, sep