数据库理论中，数据的独立性一般可分为 【4】 和逻辑独立性。

. Tom said he had been working for three hours, ________, evidently, was untrue.

A．he

B．this

C．which

D．it

树是结点的集合，它的根结点数目是

A．有且只有1个

B．1或多于1个

C．0或1个

D．至少2个

— The boss said we had only three days to finish the work.

— Don't worry.We have already________two thirds of it. [      ]

A. got down  

B. got through

C. given in  

D. given away

假设消费者的嗜好转向苹果消费。以下哪一种说法是对苹果市场上这个事件影响的正确描述？（）

A.苹果的需求增加，苹果的供给量增加。

B.苹果的需求和供给都增加。

C.苹果的需求量和供给都增加。

D.苹果的需求增加，苹果的供给减少。

E.苹果的需求量减少，苹果的供给增加。

ZD6系列电动转辙机自动开闭器动接点与静接点组的接触深度，两侧相差不大于（）。

A.1MM

B.1.2MM

C.1.5MM

D.2MM