The analysis of item A reflects the poet's determination to make meritorious deeds by killing the enemy and his belief in winning, while items C and D are descriptions of natural landscapes, which cannot reflect the sense of hardship and can be ruled out. Item B reflects Du Fu's view that the capital is dilapidated and the city is in trouble. Although the mountains and rivers remain the same, there are lush grass and green trees everywhere, which reflects the poet's bleak mood of encountering an Shi rebellion and meets the requirements of the topic. So item B is the correct answer.