The mass extinction occurring today differs from those that occurred in the past because

a.) it is due mainly to the actions of one species
b.) species are no longer evolving
c.) it is occurring globally
d.) niches are not being filled

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.