Question

complete the sentence.
snakes propel themselves along the ground with s - shaped movements, using both their ventral scales—which grip the ground like the tread on a tire—and ______.
by contracting their muscles
coordinated muscle contractions

Explanation:

The sentence uses parallel structure with "using both their ventral scales... and [parallel structure]". The first item is a noun phrase with a descriptive clause, so the second item should match the noun phrase structure for grammatical consistency. "coordinated muscle contractions" is a noun phrase, while "by contracting their muscles" is a prepositional phrase that breaks the parallelism set up by "using both X and Y".

Answer:

coordinated muscle contractions