Q:I want to test the ability of students to read a passage and answer questions about it. Could you please come up with a good question for the passage "There were guilds of dyers who specialized in red in Venice and other large Europeans cities. The Rubia plant was used to make the most common dye; it produced an orange-red or brick red color used to dye the clothes of merchants and artisans. For the wealthy, the dye used was Kermes, made from a tiny scale insect which fed on the branches and leaves of the oak tree. For those with even more money there was Polish Cochineal; also known as Kermes vermilio or "Blood of Saint John", which was made from a related insect, the Margodes polonicus. It made a more vivid red than ordinary Kermes. The finest and most expensive variety of red made from insects was the "Kermes" of Armenia (Armenian cochineal, also known as Persian kirmiz), made by collecting and crushing Porphyophora hamelii, an insect which lived on the roots and stems of certain grasses. The pigment and dye merchants of Venice imported and sold all of these products and also manufactured their own color, called Venetian red, which was considered the most expensive and finest red in Europe. Its secret ingredient was arsenic, which brightened the color."?
A:
What was the basic ingredient that three of the most expensive dyes had in common?