题目

For several years, Americans have enjoyed teleshopping-watching TV and buying things by phone. Now teleshopping is   1 in Europe. In a number of European countries, people   2 rurn on their TVs and shop for clothes,jewelry,food,and many other things. Teleshopping is becoming   3 in Europe. The biggest Swedish company sells different kinds of things on TV in European countries,and in one year it can    4  about $100 million. In France there are two teleshopping  5 ,and the French spend about $20 million a year buying things 6 these channels. InGermany,teleshopping was only one chann在物理学中，把人们由听觉感受到的声音的高低称为___________，听觉感受到的声音的强弱称为___________；即使是高低、强弱都相同的声音，听起来也不完全一样，可见声音除了高低、强弱这些特征外，还有第三个特征，即___________。