I’m confused on ur multiple choose “answers”
        
             
        
        
        
Answer:
Option D, A subject is observed both before and after two exposures to the treatment.
Explanation:
In A-B-A-B design the alphabets have following meaning
a)	First A – It is the baseline value or the value measured before treatment
b)	First B – It is the treatment measurement
c)	Second A  - It is the withdrawal of treatment  
d)	Second B – It is the reintroduction of treatment  
The second A is known as the reversal phase in which the intervention is withdrawn to see if the target behavior returns to the baseline behaviour
Thus , there are two measurements involved one before the intervention and second after the intervention
Option D is correct
 
        
             
        
        
        
Answer:
D)
Explanation:
Turtles eat the grass, grass is needed for fish breeding. If there are more turtles, they will eat more grass and there will obvious be less of it. If there is not enough grass, other populations of fish will leave because they can't breed.