Question

read the sentence from the passage.
why did a once - common bird almost disappear?
which conclusion can be drawn about trumpeter swans from the sentence?
at one time, there was an abundance of trumpeter swans.
trumpeter swans look similar to many other types of birds.
experts are confused about why trumpeter swans moved away.
the reason for the decline in the trumpeter swan population is still unknown.

Explanation:

The key word in the sentence is "once - common". "Once - common" implies that at a certain time in the past, the bird was abundant. Option A directly relates to this implication. Option B about looking similar is not related to the "once - common" part. Option C about experts being confused is not indicated by the sentence. Option D about the reason being unknown is also not relevant to the "once - common" description.

Answer:

A. At one time, there was an abundance of trumpeter swans.