题目

用于火灾报警的离子烟雾传感器如图所示．在网罩Ⅰ内有电极Ⅱ和Ⅲ ， a、b两端接电源，Ⅳ是一小块放射性同位素镅241，它能放射出一种很容易使空气电离的粒子．正常情况下镅241放射出的粒子使两个极板间的空气电离，在a、b间形成较强的电流；发生火灾时，大量烟雾进入网罩Ⅰ内，烟尘颗粒吸收其中的带电粒子，导致电流发生变化，从而报警．下列说法中正确的是（ ） A . 镅241射出的是α粒子，有烟雾时电流增强 B . 镅241射出的是α粒子，有烟完形填空 阅读下面短文，掌握其大意，然后从以下各题所给的四个选项(A、B、C和D)中，选出最佳选项。 　　With a heavy heart Sally arrived early on the morning of the auction (拍卖). It was almost a year since her dear grandmother had passed away in this house at the age of ninety-two. 1 the family business had failed and left them with heavy debts. So the bank had 2 the family to have an auction sale to repay their debts. 　　 3 the past several weeks, she had managed to save $ 100 in order to bid (出价) on the 4 . Surely this would be enough, she had thought. Now the 5 moment had arrived. Looking around, Sally was surprised to see so many people had 6 , some from far away. 　　As the auction began, Sally waited. Finally, the time came to bid on the quilt. She held her 7 tightly and listened. The first bid was $ 50. She was 8 . It was so high. Quickly other people bid and soon the bidding 9 $85. Sally cried out “$100!” For a moment there was silence. Hope 10 her. She glanced again at the quilt, thinking how 11 her grandmother would have been with her 12 . 　　Just then, from the back, another person shouted “$125!” She had lost her 13 . Suddenly she couldn't help wondering why so many people 14 be interested in that old, torn, dirty quilt. The quilt was sold for $ 500 to a complete 15 . 　　After the sale, she went to the counter to pay for the few items that were now hers. The woman at the cash desk handed her the quilt. “There must be some 16 . Someone else got it.” She said. A hand - written 17 was pinned to the quilt, which read: 　　Dear Madam, 　　I noticed you 18 the quilt. Clearly, it was 19 to you. Please accept this quilt as my gift. I don't expect repayment from you, but do a similar favor in the future for a stranger who 20 to cross your path. 1． [　　] A．NaturallyB．Unfortunately C．LuckilyD．Strangely 2． [　　] A．forbiddenB．waited C．expectedD．forced 3． [　　] A．WithB．At C．ByD．Over 4． [　　] A．houseB．pillow C．furnitureD．quilt 5． [　　] A．strangeB．necessary C．bigD．curious 6． [　　] A．turned outB．turned up C．turned backD．turned on 7． [　　] A．purseB．picture C．bookD．quilt 8． [　　] A．excitedB．calm C．worriedD．shocked 9． [　　] A．roseB．climbed C．reachedD．won 10． [　　] A．toldB．helped C．filledD．gave 11． [　　] A．pleasedB．anxious C．angryD．familiar 12． [　　] A．at this momentB．ever since C．long beforeD．soon after 13． [　　] A．thoughtB．chance C．methodD．fortune 14． [　　] A．couldB．might C．mustD．would 15． [　　] A．relativeB．customer C．friendD．stranger 16． [　　] A．reasonB．secret C．mistakeD．fun 17． [　　] A．checkB．wallet C．addressD．note 18． [　　] A．admiringB．touching C．watchingD．holding 19． [　　] A．wonderfulB．various C．prettyD．precious 20． [　　] A．usedB．stops C．happensD．comes