How many letters does the top answer for “a flammable gas obtained by cracking petroleum; used in organic synthesis” have?

The most common answer is propene (7 letters).

Crossword clue: a flammable gas obtained by cracking petroleum; used in organic synthesis

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

propene 7 letters wn-def

a flammable gas obtained by cracking petroleum; used in organic synthesis

propylene 9 letters wn-def

a flammable gas obtained by cracking petroleum; used in organic synthesis