The main reason for the difference in the mass extinction occurring today and previously is the pace of the current extinction. The extinction is taking place globally and involves hundreds of species at the same time. In less than 50 years, we have lost numerous species. In contrast to this, previously during the mass extinction, only a few species were lost, and other managed to survive. But, not the status is opposite due to the magnitude of the extinction. 
        
                    
             
        
        
        
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(D) Nonpoint source pollution is difficult to regulate, because its source is difficult to track.
 
        
             
        
        
        
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The nitrogen cycle. Animal and plant waste decomposes and adds nitrogen to the soil, which bacteria convert into a form that plants use to grow. Animals eat the plants, which completes the cycle.