2.物理性质：单质硅有　　 和　 　两种，晶体硅结构类似与金刚石，属　　 晶体，硬度大，熔点高，是良好的　　　 材料。是人类将太阳能转换为电能的常用材料。

1．存在：在地壳中硅的含量仅次于____。在自然界中以　　 和　　 的形式存在，是构成矿物和岩石的主要元素。原子结构示意图为_______________。即不易失电子和也不易得电子，主要形成　　 价的化合物。

5.Steve and Mark are both studying in the Chinese Department of China's Renmin University. They want to do some research on Chinese ancient literature.

答案：1-5　DBECA

4.Edward is a senior student in Beijing Foreign Language University. He likes traveling very much and has made up his mind to work as an interpreter for some joint­venture enterprises (合资企业).

3.Lora and Peter， visiting professors from Australia， are both crazy about Chinese traditional culture. At weekends they like to call on Chinese families to learn about Chinese festivals as well as their history.

2.Simon comes from Egypt. He is now studying in Beijing Art Academy. He shows great interest in Chinese ancient characters. Now he wants to know much about it.

Valentine's Day is the time of the year when couples show their love for each other by sending cards，flowers and chocolates. Its origins are 　1　(clear)and hidden in the river of time. Nobody knows exactly 　2　 St. Valentine was，although some historians suggest he was 　3　 Roman killed in the third century AD by a Roman Emperor. It is said that the first 　4　 (record)Valentine's card was sent by the imprisoned Duke of Orleans in 1415.　5　 is believed that he tried to find comfort from his imprisonment by writing love poems to his wife.

Now，Valentine's Day 　6　(celebrate)in many countries around the world. However， the traditions often differ from place to place. In Japan，for example，it is normal for the woman to send chocolates to the man，　7　 in Korea April 14th is known 　8　“Black Day” and is a time when the unfortunate men who received nothing on Valentine's Day gather to eat noodles and show sympathy for each other.

Technological 　9　(develop)have also played their part 　10　 keeping Valentine's Day relevant in the 21st century. Valentine's e­cards have been very popular in recent years.

答案：1.unclear　2.who　3.a　4.recorded　5.It　6.is celebrated　7.while/but　8.as　9.developments　10.in

12.(1) v_t为末速度,t为运动时间,2表示物体运动的加速度为2m/s²方向与初速度相同,4表示物体运动的初速度为4 m/s　

(2)图象略