The expression "Están creciendo mucho" is grammatically correct.
The expression "Están creciendo mucho" is formed by a tacit pronoun (Ellos), verb "ser", an auxiliar verb in gerund and an quantity adverb. The expression is grammatically correct since it observes the following grammatical construction:
Pronoun (optional) + verb "ser" + auxiliar verb in gerund form + quantity adverb.
(Ellos) están creciendo mucho.
In other words, the expression "Están creciendo mucho" is grammatically correct.
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