What is the definition of "periodically" as used in this

Last updated: 7/9/2022

What is the definition of "periodically" as used in this selection? Rhythmic, or phasic, smooth muscle contracts periodically. It spends most of the time in a relaxed state. Tonic smooth muscle remains contracted for the majority of the time and only relaxes periodically. A for long stretches B from time to time C randomly but frequently D using an exact schedule