Directions (Qs. 15 to 21): Read the passage andanswer the following questions. Have you ever heard someone usethe phrase "once in a blue moon?" People use this expression todescribe something that they do not do very often. For example, someone mightsay that he tries to avoid eating sweets because they are unhealthy, but willeat chocolate "once in a blue moon" or someone who does not usuallylike 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 is thatthe 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 thecolour.As the moon travels around theearth, it appears to change shape. We associate certain names with certainshapes of the moon. For example, when we can see a small part of the moon, it iscalled a crescent moon. A crescent is a shape that looks like the tip of afingernail. When we cannot see the moon at all, it is called a new moon. Whenwe can see the entire moon, it is called a full moon. Usually, there is onlyone full moon every month. Sometimes, however, there will be two full moons inone month. When this happens, the second full moon is called a "bluemoon."Over the next 20 years, there willonly be 15 blue moons.As you can see, a blue moon is avery rare event. This fact has led people to use the expression "once in ablue moon" to describe other very rare events in their lives. As used in the final paragraph, which is the best antonym for rare?

• A. Common
• B. Strange
• C. Colourful
• D. Infrequent