A、The depth of the holes in vegetables. B、The size of the vegetables. C、The shape of the vegetables. D、The amount of water conta

　　Do you consider yourself an expert at playing with your food? Even if you can toy with dinner on a higher level, (16) you might be impressed with the work of two brothers—instead of eating their vegetables, the two American artists use them to create working musical instruments. Vegetables and music were a large part of the brothers’ lives when they were growing up. Their father was a music teacher who raised them on a vegetable farm in central America. He taught them how to play traditional musical instruments and skills they now use to make a living as adults. The two brothers appear regularly on American television, (17) playing a wide variety of music on their vegetable instruments. Each musical appearance takes a lot of preparation. (17) In addition to practicing the songs they will play, the brothers also have to make a new set of instruments out of fresh vegetables for every concert. "The vegetables have to be solid and hard," the elder brother told us. "We can’t use vegetables left over for days. They are too soft to be played." (18) He also says they choose vegetables with lots of water in them, as those make the best instruments. The brothers’ skills are also important to the instruments’ sound. "The deeper the hole is, the lower the pitch is. The shallower the hole is, the higher the pitch is," said the younger brother. "The size of the holes also matters to the quality of the sound." However, the brothers didn’t say whether they eat their musical creations when the concerts are over.
　　16. Why can we be impressed by the two American artists?
　　17. What can we learn about the brothers from the passage?
　　18. What is the most critical factor for a vegetable instrument?