Nasreddin had a shed(棚子)behind his house. It had no lights in it. One night he went out to the shed to get his ladders(梯子)and lost his ring(戒指)there. He left the ladder, went out into the street and began to look around.
One of his friends saw him in the street outside his house, and said to him, " Hello, Nasreddin, what are you looking for"
"My ring , "answered Nasreddin. "It fell off my finger. It is a silver(银的)ring with a red stone in it." "Oh, yes. "said the friend. "I remember it. I will help you to look for it. Where did you lose it"
"In my shed."
"But why don’t you look for it there"
"Don’t be foolish! It is quite dark in my shed, how can I find my ring there Here there is light from the lamps in the street.\
Nasreddin had lost his ______.
A. ring B. silver C. money
u（n）的Z变换，（）。
，极点为z=0