Sludge in the extract means?

UNSEEN POETRY AND PROSE

Read the poem and answer the question
Bent-double, like old beggars under sacks,
Knock-kneed, coughing like hags, we curse through sludge
Till on the haunting flares we turned our backs,
And towards our distant rest began to trudge,
Men marched asleep, many had lost their boots,
But limped on, blood-shed. All went lame, all blind;
Drunk with fatigue; even deaf to the hoots
Of gas-shells dropping softy behind.

A. water
B. fire
C. snow
D. mud

Correct Answer:

Option D – mud