The Dust Bowl Days is your answer
The Dust Bowl was a period in history known for the destruction of the plain lands (Central USA and westward), which led to large amounts of people to go to California in hopes of starting a new life.
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The correct answer is D, because "their" signifie that they are having "their" conversation, and that they own the conversation. C is incorrect because "there" describes where, B is incorrect because "they" describe who, and A is incorrect because "they are" doesn't grammatically or sensibly fit in, so the answer is D.
<span>The conference that was held on January 20, 1942 to finalize plans for the Final Solution was known as the "Wannsee Conference," although there were discussions prior to this as well. </span>
