Answer:
The horses developed an oxygen debt when they exercise because they suffered from Exercise-Induced Pulmonary Hemorrhage (EIPH).
Explanation:
A temporary shortage of oxygen in the body tissues during physical exertion such as exercise or a sudden burst of activity is referred to as oxygen shortage. 
Horses (compared to humans) are uniquely designed to experience little or none of this.
When the body takes up oxygen, it reacts with glucose to produce energy. This is called Aerobic respiration.
There is also another procedure that the body uses to produce energy. This occurs when glucose is broken down in the cells. After this happens, lactic acid is created but cannot be stored in the body and must be expelled. 
This interval when energy is produced without the presence of oxygen is called "oxygen debt".
When a horse is fit, its breathing quickly returns to normal. The longer the time taken for the breathing to return to normal, the less fit the horse is. 
One of the factors that can reduce the fitness of a horse is the Exercise-Induced Pulmonary Hemorrhage (EIPH).
This occurs when blood vessle in the lung ruptures causing blood to bleed into the airways. So blood occupies space where oxygen should have. This reduced the amount of oxygen delivered to the blood stream. Hence the oxygen debt takes longer to be repaid.
Cheers
 
        
             
        
        
        
The principles of probability can be used for B. Predicting the traits of the offspring of genetic crosses. 
Answer: B. Predict the traits of the offspring of genetic crosses.
        
                    
             
        
        
        
I would say the answer is A
        
             
        
        
        
A galaxy where mainly stars and planets form around one or more spiral arms. (usually more?)
        
                    
             
        
        
        
Answer:
O founder's effect
Explanation:
A founder effect can be defined as the loss of genetic variation when a new population is established from a few individuals. This process is known to increase the frequency of particular gene variants (alleles) at different <em>loci</em> when they are selectively neutral (or nearly neutral), and thereby such genes are fixed by genetic drift (i.e., through the random sampling of founder individuals). Interestingly, it has been discovered that the majority of South American and Central American Indians are nearly exclusively in the O blood group, which has been further associated with random genetic drift and a founder effect.