Pyroclastic flows primarily contain pumice, vesicular pyroclastic material, and .
a. ultramafic rock debris
b. a cloud of low-density hot expanding gases
c. low viscosity
d. lava flows volcanic mudslides called lahars

d. lava flows volcanic mudslides called lahars

Explanation:

Mudflows of volcanic eruptions are the most dangerous, and are called lahars. A lahar is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water.

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