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I have a question about a practice test!! Why is the correct answer D? I choose A.

Changes in the weather can trigger the start of the journey south, although birds in the Northern Hemisphere seem to know when it is time to migrate south before the winter. In some species at least, the changes in the length of the day cause glands in the birds' bodies to secrete hormones that produce other changes, which ready the birds for the long flight south. At this time fat starts to accumulate under the skin, and this provides a store of energy for the long flight when they will be expending more calories flying than they can obtain during their brief rest stops.
According to paragraph 2, what are the results of changes in the weather?

A, The change in the length of the day

B, The secretion of hormones by the birds

C, The expenditure of calories

D, The onset of migration
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2 Answers

The point of the paragraph is the onset of the activity by birds in response to the weather; in some species the length of the day triggers the hormonal change. While weather might change the day but that is not the main point and therefore, A is incorrect.
B and C are not the direct outcome of the change of the weather and  therefore, those are incorrect.
Migration of birds is the direct outcome of the weather, it is included in statement D and therefore, D is the correct answer.
Hope it helps.
Dear Saori,
The change in the weather does not cause the length of the day to change.  If anything, its the other way around.  Trust me: D is the right answer.