Over the centuries, henna tattoos 16 used in weddings and holiday celebrations because they are thought to carry blessings from the gods.
答案：(B) have been
現在完成式被動語態 have been，配合 Over the centuries（在幾世紀以來，持續到現在），表示指甲花刺青一直被用於婚禮節慶；were being（過去進行式，不符合持續到現在的語意）、will be（未來式）、had to be（過去必須）都不合。
空格後面已經印好 used，所以你要填的是助動詞部分。四個選項都塞得進去、讀起來也都通，這時候唯一能定案的是句首那個時間副詞片語：Over the centuries（幾個世紀以來）。
Over the centuries 和 since 一樣，畫的是「從過去一路延伸到現在」的時間軸，所以要用現在完成式。再看語態：主詞是 henna tattoos（刺青），刺青是被人使用的，不是自己去使用別人，所以要用被動。現在完成式＋被動＝ have been ＋ 過去分詞，也就是 have been used。文內也支持這個判斷：最後一句說 the number of artists... has grown to over 150（參與的藝術家已成長到 150 人以上），用的同樣是現在完成式，整篇的時間感是「延續到今天」。
were being 是過去進行被動，把事情鎖在過去某個時段，與「幾世紀以來一直如此」衝突；will be 是未來式，但這是既有的傳統不是將來才要發生的事；had to be 是「過去必須被……」，多了一層「不得不」的意味，文章講的是文化習俗，沒有被迫的語氣。
over the centuries（幾個世紀以來）、have been Vpp（現在完成式被動，一直被……）、date back to（可追溯到）、be thought to V（被認為……）、carry blessings（帶來祝福）、for centuries（數百年來）。
The art and tradition of using all-natural paste from the henna plant to create temporary henna tattoos goes back to ancient Asia and the Middle East. Over the centuries, henna tattoos 16 used in weddings and holiday celebrations because they are thought to carry blessings from the gods.
Today, the ancient art form has found a new 17 in the hands of contemporary artists. Henna Heals, an artist group based in Canada, uses henna tattoos to help women suffering from cancer feel confident and beautiful again. They draw elegant henna crowns on the women's bare heads to help them 18 the loss of their hair after cancer treatment.
Featuring floral patterns, religious symbols, and messages of hope, the painted henna crowns are amazing substitutes for the hats and wigs that the cancer patients would 19 use to cover their heads. Many women report that the henna crowns really work as a healing device, helping them build a more 20 self-image at a time when they are vulnerable and depressed. According to Frances Darwin, founder of Henna Heals, the number of artists now involved in the project across Canada and the US has grown to over 150.