Look at these examples:
Questions 1-2
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Parasites such as viruses, bacteria, and fungi--the causes of most disease--specialize in breaking into cells, either to eat them, as fungi and bacteria do, or, like viruses, to subvert their genetic machinery for the purpose of making new viruses. Either way, they must get into cells. To do that, they employ protein molecules that fit into, or bind to, other molecules on cell surfaces. The struggle between parasites and their hosts is all about these binding procedures.
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1. The word "their" in line 3 refers to
(A) parasites
(B) viruse
(C) fungi
(D) cells
2. The word "they" in line 4 refers to
(A) parasites
(B) diseases
(C) cells
(D) protein molecules
The correct answer to question 1 is (D), cells, because it is the cell's genetic machinery that viruses subvert. Parasites, viruses, and fungi all specialize in breaking into cells.
You can check the answer by replacing their with cell's in the sentence.
The correct answer to question 2 is (A), parasites. It is parasites such as viruses, bacteria, and fungi that specialize in breaking into cells. Parasites is the subject of the first sentence, and they is the subject of the next the two sentences (line 4). This parallel position is a structural clue that helps you see that they refers to parasites.
You can check the answer by replacing they with parasites.