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问题 选择题

元素的化学性质取决于原子的（　　）

A．最外层电子数

B．质子数

C．中子数

D．核电荷数

答案

当原子的最外层电子少于4个时，容易失去电子；多于4个少于8个时，容易获得电子；最外层为8个电子时，元素的化学性质最稳定．元素的化学性质主要决定于原子的最外层电子数，与质子数、中子数、核电荷数无关，所以A正确，B、C、D错误．

故选A．

单项选择题

Some of the keys have a special use. There are referred to as command keys. The 3 most common are the Control or (66) , Alternate or (67) and the Shift keys though there can be more (the Windows key for example or the Command key). Each key on a standard keyboard has one or two characters. Press the key to get the lower character and hold Shift to get the upper. Disk Drives - All (68) need a drive to get information off - or (69) - and put information on the disk - or (70) . Each drive is designed for a specific type of disk whether it is a CD, DVD, hard disk or floppy. Often the term ’disk’and ’drive’are used to describe the same thing but it helps to understand that the disk is the storage device which contains computer files-or software - and the drive is the mechanism that runs the disk.

A．read

B．write

C．speak

D．tell
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单项选择题

下面哪种铸造缺陷是由于冷却不均匀产生的应力使金属表面断裂而引起的？（）。

A.疏松

B.热裂

C.气孔

D.夹杂
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