Question

ancient mesoamerican rubber
they also needed to know how to create specific kinds of rubber - bouncy,
wear - resistant, adhesive, and so on - for particular uses.
today the largest source of latex rubber in the world is a rubber plantation in
liberia. but in 1999, a team from the massachusetts institute of technology,
drawing on historical records, archaeological evidence, and hands - on
experiments, concluded that ancient mesoamerican rubber makers likely
mixed latex with the juice of a type of morning glory vine. stirring the warmed
liquids together causes a chemical interaction and, within fifteen minutes, the
mature coagulates into solid rubber. in a later study, the team found that
varying the proportions of them changes the properties of the product. an
equal mix of latex and juice, for example, produces a resilient rubber suitable
for game balls. a three - to - one mix, nonetheless, creates a tougher rubber
which was ideal for sandal soles.
more research, including further analysis of archaeological artifacts, are
needed to fully verify the teams conclusions. but the findings so far suggest
that ancient mesoamericans were skilled materials scientists who were able
to regulate resources in a sophisticated way to meet their everyday needs.

Explanation:

In the sentence “varying the proportions of them changes the properties of the product”, “them” is ambiguous. The context is about varying the proportions of latex and juice (the two substances mixed). “The two liquids” (Option B) clearly refers to these two substances. “NO CHANGE” (Option A) with “them” is too vague. “those” (Option C) is also not as specific as “the two liquids”. “it” (Option D) is singular while we are referring to two substances.

Answer:

B. the two liquids