13. The theme of Sandburg's poem "Grass" is A. war. B. forgetfulness. C. freedom. D. peace.

The theme of Sandburg's poem "Grass" is primarily about the passage of time and the natural process of forgetting. The poem personifies grass as a force that covers up the remnants of war and the memories of those who have died, suggesting that over time, the grass will erase the evidence of conflict and suffering. Therefore, the correct answer is B. forgetfulness.