Which word signals a cause? O so O then O since O therefore​

It could be either so, since, or therefor. they all apply

dr. kings dream speech

explanation:

it was late in the day and hot, and after a long march and an afternoon of speeches about federal legislation, unemployment and racial and social justice, the rev. dr. martin luther king jr. finally stepped to the lectern, in front of the lincoln memorial, to address the crowd of 250,000 gathered on the national mall.

the positive connotations of the words "pretty," "quiet," and "silence" give the excerpt a tone of nervousness. i think its this