阅读下面材料, 在空白处填入适当的内容（每空1~3词）或括号内单词的正确形式。将答案写在答题纸的相应位置。
    I am writing a paper for my English class, and I need some ideas. What do you think is the most useful invention of all time? Thank for your (think).
    Speaker 1: There are many —the phone, the car, the airplane, these are not my choices. For an everyday, useful invention, I vote for the air conditioner. I live in Hone Kong, and during the summer, the temperature reaches 35 degrees Celsius or more. It's (terrible) hot! Without air conditioning, people here cannot work or study. A lot of people might also get sick cool air.
    Speaker 2: (useful) inventions of all time are the Internet and the World Wide Web. We can now communicate with people and do business faster. I own a clothing store in Mexico City, and later I set up a website by (I). Now, people from all over the world can buy my clothing using my website. I can keep in touch with friends and family in Mexico and around the world using e-mail. It's faster than regular mail, and it's cheaper than using the phone!
    Speaker 3: I agree with the（two） speaker—the Internet is useful. But, in my opinion, there is an ‘invention' is more important—electricity. Of course, this isn't man-made invention, but people like Benjamin Franklin and Alessandro Volta learned how (use) it. Without electricity, many of the world's most important inventions would not work.