28. Homelessness occurs when a part of the population does not have any fixed place to call home.
  -> C. fixed place

29. What does the word "This" refer to?
  -> B. Homeless

30. If you have a lot of free time, you might go on an extended trip to help the homeless.
  -> A. go on an extended trip

31. The phrase "sign up" can best be replaced by:
  -> D. register

32. Most cities have a mission of offering temporary shelter.
  -> B. temporary shelter

28. Homelessness occurs when a part of the population does not have any fixed place to call home. "Fixed place" means a permanent or stable location, which is the opposite of homelessness where individuals lack a stable residence.

29. In the context of the passage, "This" refers to the issue of homelessness mentioned earlier in the sentence. It's indicating that homelessness is a problem that authorities are trying to deal with.

30. The sentence suggests that if you have a lot of free time, one way you could help the homeless is by going on an extended trip to assist them. This option emphasizes the idea of dedicating an extended period to helping, which is captured by "go on an extended trip."

31. "Sign up" means to officially agree to participate in something or to join a group or activity. The best replacement for this phrase is "register," which also implies officially enrolling or joining.

32. The sentence talks about the missions in cities that offer temporary shelter to the homeless. "Temporary shelter" refers to short-term accommodation provided for those without homes, which aligns with the purpose of the missions mentioned in the passage.