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 the structure "using both X—and Y", requiring parallel grammatical structure. The first element is a noun phrase ("their ventral scales"), so the second element must also be a noun phrase to maintain parallelism. "Coordinated muscle contractions" fits this, while "by contracting their muscles" is a prepositional phrase that breaks the parallel structure.

Answer:

coordinated muscle contractions