Sentence Correction

1. Cars cost more than bicycles - Is this ok? or do we have to say. (Looks like cost here is verb)
Cars cost more than bicycles do

We are comparing cost of cars with cost of bicycles,so how do we say it.

I am confused when we should add verb at the end and we should not. Sometimes adding verb is wordy and sometimes not concise ,so we cannot omit this concept.

2. To meet the rapidly rising market demand for fish and seafood, suppliers are growing fish twice as fast as their natural growth rate, cutting their feed allotment by nearly half and raising them on special diets.
(A) their natural growth rate, cutting their feed allotment
(B) their natural growth rate, their feed allotment cut
(C) growing them naturally, cutting their feed allotment
(D) they grow naturally, cutting their feed allotment
(E) they grow naturally, with their feed allotment cut


In D and E the comparison is between

suppliers are growing fish twice as fast as fish grow naturally....

Had the comparison been suppliers can we omit verb?
Indian suppliers are growing fish twice as fast as American suppliers--

Is it like if we compare subjects we can omit the second verb
and if we compare verbs then we should have the second verb .
Can someone help