ridged or grooved.
The corrugated surface of the dirt road meant that the passengers in the bus were shaken to bits at the end of the journey.
When it has a subject, verb, punctuation, capitalization, a predicate, and a complete thought. A sentence is a sentence when it is handed down by a judge (or a magistrate).
This is one of a series of questions copied from Penn-Foster's "Practical English" quiz.There is no answer here.