In the gutters under the eaves and between the shingles of the roofs, a white granular powder still showed a few patches; some weeks before it had fallen like snow upon the roofs and the lawns, the fields and streams. which best explains how the phrase “white granular powder” supports the author’s purpose of calling attention to the environment? the powder is an unnatural substance. the powder has a texture like sugar. the powder looks like freshly fallen snow. the powder is used in house construction

The powder is an unnatural substance.

Explanation:

The term "white granular powder" means that this is an unnatural, unknown, unknown substance that is unknown. If this were a natural substance, the name of the substance would probably have been announced. As this substance is not natural, the excerpt shows some concern about its effects on the roofs and lawns, fields and streams.

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.