I certainly never said anything about comparing clauses and verbs, and all of your comparison sentences are fine. We can be flexible with comparisons involving verbs that don't take objects (what are called 'intransitive verbs' in grammar jargon), because there's no risk of confusing what is being compared. If you want to think about comparisons that are analogies of the one in the original sentence in this thread, you'd need to consider verbs that take objects, because it's in that situation where comparisons can become ambiguous: "Paul likes chocolate more than his manager", for example, could mean Paul likes chocolate more than he likes his manager, or it could mean that Paul likes chocolate more than his manager does. In those situations we often need to be careful about how we construct the comparison if we want to avoid ambiguity (which we want to avoid in GMAT SC).

In the original sentence, we're talking about "growing fish", so there's an object involved, and those comparisons need special attention. If you take answer C, which I think is the choice you were asking about, and interpret it in either of the possible ways one can interpret a comparison with an object (as above, in the Paul/chocolate/manager sentence), you get one of these two sentences:

original: Suppliers are growing fish twice as fast as growing them naturally

if we were comparing two subjects: Suppliers are growing fish twice as fast as growing them naturally (is growing fish)
if we were comparing the first subject in two respects: Suppliers are growing fish twice as fast as (they are) growing them naturally

Neither makes any sense. We want to compare how quickly suppliers are growing fish with how quickly fish grow naturally. Only answers D and E do that.
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