giải thik nx nha!!! Have you ever heard someone use the phrase “once in a blue moon”? People use this expression to describe something that they do not do very often. For example, someone might say that he tries to avoid eating sweets because they are unhealthy, but will eat chocolate “once in a blue moon”. Or someone who doesn’t usually like to go to the beach might say “I visit the shore once in a blue moon.” While many people use this phrase, not everyone knows the meaning behind it. The first thing to know that the moon itself is never actually blue. This is just an expression. The phrase “blue moon” actually has to do with the shape of the moon, not the color. As the moon travels around the Earth, it appears to change shape. We associate certain names with certain shapes of the moon. For example, when we can see a small part of the moon, it is called a crescent moon. A crescent is a shape that looks like a tip of a fingernail. When we cannot see the moon at all, it is called a new moon. When we can see the entire moon, it is called a full moon. Usually, there is only one full moon every month. Sometimes, however, there will be two full moons in one month. When this happens, the second full moon is called a “blue moon”. Over the next 20 years, there will only be 15 blue moons. As you can see, a blue moon is a very rare event. This fact has led people to use the expression “once in a blue moon” to describe other very rare events in their lives. 41. Which would be a good example of someone doing something “once in a blue moon”? A. Mary likes to go to the mountains every weekend. B. Tom rarely remembers to take out the trash. C. Cindy hates washing the dishes. D. Mary sometimes forgets to do homework. 42. When does a blue moon happen in nature? A. When there are two full moons in a month. B. When the moon has blue color. C. When we can only see a small part of the moon. D. When we cannot see the moon at all. 43. According to the passage, it can be understood that which of the following sentences does not contain the expression? A. Thomas has lost his mind B. An apple a day keeps the doctor away C. I’ll mow the grass after I finish my homework. D. It’s never bad time to start something new 44. According to the passage, what is another example of something that has crescent shape? A. your thumb B. a distant star C. the letter “C” D. the letter “H” 45. The author states that “Over the next 20 years, there will only be 15 blue moons.” This means that over the next 20 year, a blue moon will happen …… A. once a year B. less than once a year C. more than once a year D. Not enough information is provided