把A、B、C、D 4块金属片浸入稀硫酸中，用导线两两相连组成原电池．若A、B相连时，A为负极；C、D相连时，D上产生大量气泡；A、C相连时，电流由C经导线流向A；B、D相连时，电子由D经导线流向B，则此4种金属的活动性由强到弱的顺序为（ ）
A．A＞B＞C＞D
B．A＞C＞D＞B
C．C＞A＞B＞D
D．B＞A＞C＞D
A、B相连时，A为负极，活泼金属作负极，所以A的金属活动性大于B；
C、D相连时，D上产生大量气泡，D上得电子发生还原反应，所以C上失电子发生氧化反应，故C的金属活动性大于D；
A、C相连时，电流由C经导线流向A，电流由正极流向负极，所以A作负极C作正极，A的金属活动性大于C；
B、D相连时，电子由D经导线流向B，电子的流向与电流的流向相反，由负极流向正极，所以D是负极，B是正极，D的金属活动性大于B，
所以A、B、C、D 4块金属的活动性由强到弱的顺序为A＞C＞D＞B．
故选B．
| 完形填空 | |||
| A boy was walking home from school when he saw a large, tempting (诱人的) apple on one of the branches of an apple tree hanging out over a tall fence.The boy wasn't much of a fruiteater,__1__ a bar of chocolate if given the choice,__2__, as they say, the forbidden fruit can be tempting.Seeing the apple, the boy wanted it.The more he looked at it, the__3__ he felt and the more he wanted that apple. He stood on tiptoe (脚尖) , __4__ as high as he could, but even at his tallest __5__ he was unable to touch it.He began to __6__up and down, as high as he could, at the __7__ of each jump stretching his arms to get the apple.Still it remained out of __8__. Not giving up, he thought, if only he had something to __9__ on.His school bag wouldn't give enough height and he didn't want to __10__ the things inside, like his lunch box, pencil case, and Gameboy. Looking __11__, he hoped he might find an old box, a rock, or, __12__ luck,even a ladder, but it was a tidy neighbourhood and there was nothing he could use. He had tried everything he could think to do.__13__ seeing any other choices, he gave up and started to walk __14__.At first he felt angry and disappointed thinking about how hungry he had become from his __15__,and how he really wanted that apple.The more he __16__ like this, the more unhappy he became. __17__, the boy of our story was a pretty smart guy, even if he couldn't always get what he wanted.He started to say to himself, "This isn't __18__.I don't have the apple and I'm feeling miserable as well.There's __19__ more I can do to get the apple-that is unchangeable-but we are supposed to be able to __20__ our feelings.If that's the case, what can I do to feel better?" | |||
| ( )1．A.preferring ( )2．A.so ( )3．A.sadder ( )4．A.expanding ( )5．A.strength ( )6．A.jump ( )7．A.tip ( )8．A.hope ( )9．A.put ( )10．A.break ( )11．A.up ( )12．A.for ( )13．A.After ( )14．A.back ( )15．A.wishes ( )16．A.thought ( )17．A.Therefore ( )18．A.skilful ( )19．A.something ( )20．A.change | B．offering B．then B．angrier B．stretching B．length B．look B．stage B．hand B．stand B．shake B．forward B．with B．Through B．away B．beliefs B．imagined B．However B．cheerful B．anything B．express | C．receiving C．but C．hungrier C．swinging C．range C．walk C．top C．sight C．get C．take C．down C．on C．Without C．up C．efforts C．tried C．Moreover C．harmful C．everything C．forget | D．allowing D．or D．tastier D．pulling D．height D．glance D．level D．reach D．hold D．strike D．around D．of D．Upon D．down D．goals D．claimed D．Otherwise D．helpful D．nothing D．describe |