能明确地和定量地表达病因、宿主和环境之间构成的疾病流行规律的研究方法是（） A.描述

     Professor Green, known to the world as a scientist, is not only absent-minded but short-sighted as well. His mind is always busy  __1__scientific problems and seldom notices what is going on __2    him.  
     One fine day recently, he went 3 a walk in the countryside, but as  __4 _he has a book in his hand.
When he went out , he began to read his book . He hadn’t gone far_ 5 __he run into a big cow and fell
down. In the  fall, he had lost his glasses, without which he couldn’t see anything. He thought he had hit
his head __6___a fat   lady. “I’m sorry, Madam.” He said politely__7 __searching for his glasses. As
oon as he had___8__, he realized his mistake.
     Soon he was fixing his mind on his book___9__and paid no attention to anything else. He had scarcely
been walking for five minutes when he fell over again, ___10__both his book and his glasses. This time 
he got very     angry, seizing his umbrella, he gave the “cow” a wild blow. Then, after finding his glasses,
he realized with horror that he made a second mistake. A large fat woman was fleeing from him in a horror.

B. boas        
B. it        
B. may         
B. went      
B. within       
B. guard     
B. looked    
B. cool      
B. so           
B. draggig     B. period       B. smiling      
B. went        
B. till        
B. looked       B. afraid       
B. left      
B. thank     
B. out       
B. blanket   

C. was         
C. whe         
C. muh          
C. arrivd       
C. in            
C. soldir       
C. jumd        
C. dirty         
C. and           
C. holdig       
C. secod       
C. lookig       
C. agred        
C. for           
C. hurrid       
C. excitd       
C. placd        
C. wrp          
C. away         C. camea      

D. birds         
D. that          
D. little        
D. came          
D. under         
D. player        
D. turned        
D. cold          
D. or            
D. catching      
D. moment        
D. shouting      
D. promised      
D. as            
D. stood         
D. angry         
D. threw         
D. help          
D. off           
D. screen