When a poet changes the meter in a poem, it forces the reader to emphasize key words. D is the correct option.
By changing the meter, there is a shift in the accentuation pattern which automatically emphasizes some parts of the lines.
Meter creates a specific rhythm to a poem. When the meter changes, the reader will have to stops or slows down while reading the poem which emphasizes any key words.
The Vietcong set up checkpoints around the nation, so that would've definitely been a literal and figurative roadblock to any refugees. Hope this helped.