单项选择题 关系型数据库是由众多二维表（基表）构成的，基表的列（字段）是基表的数据结构，一般情况下是固定不变的。

单项选择题 Fill in the blank with the most appropriate choice. The experimental investigation __________ that the addition of these fibers reduces the thermal conductivity of the composite specimen and yield a lightweight.

单项选择题 After a slow start, the plan soon took ______and was accepted by all.A.force boys to hide their emotions to be “real man”B.help to cultivate masculine aggressiveness in boys C.encourage boys to express their emotions more freelyD.naturally reinforce in boys that traditional image of a man