Over the next 20 years, there will only be 15 blue moons." This means that over the next 20 years, a blue moon will happen once a year less than once a year more than once a year not enough information is provided in sentence

In paragraph 1 the author introduces the phrase "once in a blue moon." Then the author tells us that "People use this expression to describe something that they do not do very often." This lets us know that the phrase "once in a blue moon" is an expression. In paragraph 2 the author writes, "The first thing to know is that the moon itself is never actually blue. This is just an expression." This lets us know that an expression contains information or facts that are not actually true. The sentence in (C) does not contain any facts that are not actually true. Therefore it is the correct choice.