Read the passage below carefully, and then choose the best answer:
Drawing a Line in the Sand
Artists are known to be a picky bunch. Over years of practice, they often develop a preference for certain materials. For instance, some sculptors will only work with marble, whereas some painters may insist on only using watercolour paints. Yet, only a select few have become masters working with a common material that you can find on any beach: sand. As children, we all made sand castles, but some skilled artists have taken this activity to the next level. They have created impressive, massive sand constructions in the shape of dragons, castles, and everything else you can imagine. A major benefit for many artists who build with sand is its simplicity. To get started, all you need is sand and water. However, mastering this medium takes years of practice and great attention to detail. For example, different tasks require different ratio of water to sand to get the job done. Deciding on the proper tools can also be complicated. Some artists prefer using buckets and shovels, while others are purists who refuse to use anything but their hands. Sand painting is another form of sand art. The practice dates back dozens of thousands of years and has been used by several different cultures. It involves spreading different coloured particles, which often come from crystals and rocks, onto a surface and then arranging them to make a picture. Interestingly enough, many sand paintings are designed to be temporary. Tibetan monks will spend days creating beautiful sand paintings which are blown away by winds shortly after they are finished. They view the natural destruction of their work as a symbol of impermanence in life. Meanwhile, Native Americans consider their temporary sand paintings as living creatures and they treat them with great respect while they exist. Recently, filmmakers have also discovered a way to work with sand by creating sand animation. Like other forms of animation, an artist will create one image and then capture it on camera. Then, they will make some subtle changes and then move on to the next frame and so on. After they have created and captured hundreds of images, the frames will be strung together. In the final product, it will appear as if the sand images are moving or changing shape. The next time you are at the beach, let your mind run free as you look at the shore. Who knows, you might get the urge to create some remarkable sand art of your own.
4. The word “them” in paragraph 3 refers to ___________.
