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What does "severity" mean?

"What does "against" mean?

"revelation" means............

The bleeding of people with hemophilia will last longer than of normal people if they have an accident or injury. What does "They" refer to?

It is very expensive to buy a house in the city so many people choose to rent and live in apartments or condominiums. Their prices are more affordable. What does the word "Their" refer to?

Penicillin was among the first the medicines used against the bacterial infections. However, the drug is still widely used today because the diseases have developed the resistance to it. What does "the drug" refer to?

Einstein’s theory, which describes gravity as distortions of space and time, included a mathematical element known as a Cosmological Constant. Einstein originally introduced it to explain a static universe, but discarded his mathematical factor as a blunder after it was discovered that our Universe is expanding. What does “it” refer to?

When trees are cut down for the industry, new ones are planted to replace them. Deadwood, diseased trees, and harmful bushes are removed. In replanting, foresters not only consider the suitability of the tree for climate and soil What does “ones” refer to?

Housewives once had to spend almost all of the time washing clothes, cooking food, cleaning house, keeping the home fires burning. However, scientific progress has made their lives easier and more pleasant. What does “their” refer to?

While many people these days are more conscious of privacy, especially since the revelations revealed by Edward Snowden, the vast majority of people who use services online use the same usernames from site to site. This applies especially to social networks, where it can take a lot of time and effort to create and maintain a profile. What does “This” refer to?