It means, for example if I say, "I go to the park every now and then", then it isn't a common event, but happens often enough not to be classified as a rarity.
Answer:
Marines arrive at a human settlement on a distant planet, and find that the colony has been destroyed by hostile aliens
Answer:
The answer would be C.
Explanation: The reason why is because it wraps up the paragraph neatly and doesn't leave the reader with any questions.