Which of these substances was once living material?
Ubiquitous in classrooms since the 19th century, chalk is a distinctive variety of limestone that is soft, whitish, and powdery. Chalk is a type of sedimentary rock that is formed by the accumulation of microscopic skeletons of plankton (tiny marine organisms) that lived in warm seas. When the marine organisms die, the cell degrades and the numerous hard calcitic plates that cover it descend to the seafloor. Over millions of years, the slow accumulation and compression of calcite shells has resulted in vast deposits of chalk worldwide.
The correct answer is Chalk.