I've come to the conclusion that I'm rubbish at saying “NO”.For example, last Sunday I was sitting at my desk alone，totally 1 in my own business, when a colleague (同事) of mine phoned and asked me to get involved in 2 with her. I found myself getting all caught up in her enthusiasm and didn't want to 3 her.So I said “Sure，why not?” and before the call had 4, I found myself agreeing without giving any 5 whether it was worth doing.

    We are both window-shoppers and 6 a whole day in the street. Actually, I hardly have any time to spare because I am very busy 7 my normal job—I've also got a book to write，a business to 8 and a family to look after. And then there are my other projects to 9— my voluntary work and a couple of other interesting investments(投资) that I want to 10. I'm going to have to be more ruthless(无情的)and not just agree to 11 people ask me to get involved in. In other words，I'm going to have to get 12 at saying “NO”.Usually I hate saying “NO”，because having to disappoint someone usually 13 me. Now I realize 14 I haven't learned to say “NO”. I've been feeling very 15.

    We had better do what we think is right and what we are 16 to do.Sometimes it does not seem that bad to 17 others.If I focus on the things I 18 want to be involved in，I'11 reach a much higher standard and do it quite 19. I'1l be enthusiastic and motivated(有积极性)and do something that is good for everyone and everyone will be happy.It'11 be a win-win 20. What's not good about that? So sometimes, we have to say “NO”.

蝴蝶的翅膀在2s内振动了20次，人类 ____（选填“能”或“不能”）听到蝴蝶飞行时的声音，医院里医生用B超观察母体内的婴儿情况，这利用了 ____（选填“超声波”、“次声波”或“电磁波”） 。

在图1所示的电路中，电阻R₁的阻值为5欧，滑动变阻器上标有“20Ω  2Α”字样．现有电压为6伏的电源一个，表盘如图2（a）、（b）所示的电流表、电压表各一个．