Koalas Eat Primarily Eucalyptus Leaves, Though They Are Unable to Secrete

Koalas eat primarily eucalyptus leaves, though they are unable to secrete the enzymes necessary to break down most of the fibrous material of the leaves.The most likely explanation for how they survive on this diet is that

A) their metabolism is so slow that they do not need to derive nutrients from the fibrous material.
B) by practicing coprophagia, they are able to obtain further nutrients by secreting cellulase when ingesting feces.
C) their digestive tract includes a large cecum that functions primarily to increase the absorptive area of the intestines.
D) they harbor a large microbiome in their guts that break down the majority of the fibrous material.
E) they drink large amounts of water, which is held in the gut for long periods of time to dissolve the fibrous material.