阅读理解。
Dick was having dinner with his mother and sisters one evening．
Suddenly his younger sister asked．“Why does it look like that outside?”
Everyone looked out of the window on the north side of the kitchen．
They saw the sun in a blue sky．Then．they looked out of the window on the south side of the kitchen．They saw a dark sky with big，grey clouds.
Next．they heard the tornado siren（龙卷风报警器），That meant a
tornado was coming．They all know that during a tornado，the safest
place to be is the basement（地下室）．
Luckily．they had one．They all ran quickly into the basement．
Suddenly．Dick remembered the pet cat Pete. “Mom, we forgot Pete!”
Dick yelled, “I will go upstairs and find him!” But his mother said, “No, Dick! It’s too dangerous．The tornado is coming. You h
ave to stay here.” All at once they heard the very loud noise of a very strong wind. It was so
loud that they could not even hear their own screams. Everyone was very
scared.
Just a few seconds later, it stopped. The tornado went away and everyone was safe. Dick ran upstairs and found that the house was badly damaged and Pete was gone. He began to cry and went outside. He saw that all the trees
were blown down except one. And at the top of tree, there was Pete, looking at Dick and waiting for him.
1．Why did Dick and his family go to the basement?
A．Because it was going to rain．
B．Because they heard the noise of a very strong wind．
C．Because everyone was very scared．
D．Because the safest place is the basement when a tornado
happens．
2．Why didn’t Dick’s mother let him go upstairs to find Pete?
A．Because she could not hear him．
B．Because Dick was too young．
C．Because the house Was badly damaged．
D．Because it was too dangerous．
3．Why was the loud noise Dick and his family heard when they were in the basement?
A．Screaming．
B．The wind．
C．The siren．
D．The trees．
4．Why did Dick begin to cry?
A．Because the tornado went away．
B．Because Pete was badly hurt．
C．Because Pete was gone．
D．Because Pete was at the top of the tree．
5．How many trees were blown down?
A．All except one．
B．All of them．
C．None of them．
D．Some of them．
，当
非常非常小时，就可以用
表示物体在t时刻的瞬时速度，这是应用了极限思想方法。
f(x,y)的极值点，则
点处
（其中：
）
点处df=0