Billow
"A massive billow of dust" → Correct (billow means a large cloud or mass of something rising or moving, fitting here).
"A violent billow of debris" → Correct (billow can describe a swirling mass of debris).
Cascade
"A massive cascade of dust" → Incorrect (cascade refers to flowing downward, usually for water or objects, not dust).
"A violent cascade of debris" → Correct (cascade can fit for debris but not for dust).
Torrent
"A massive torrent of dust" → Incorrect (torrent is used for rushing water or overwhelming flows, not dust).
"A violent torrent of debris" → Correct (torrent of debris could work, but not for both blanks).
Thus, Billow is the only word that fits both blanks correctly.
Hence, option (a) is the correct choice.
Study Notes
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