In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

When it interacts with ryanodine receptors, this element upregulates its own release, and this compound forms the cell plates of coccolithophores. Stimulation of Gq ("g-sub-q") opens a channel that allows for this ion's movement, and proteins that bind this ion have an EF hand motif. Cellular concentrations of this ion are regulated by IP3. Parafollicular cells of the (*) thyroid produce a compound that reduces the concentration of this ion in blood, and its intestinal absorption requires vitamin D. This ion interacts with Troponin to stimulate muscle contraction, and the lumens of sarcoplasmic reticula have high concentrations of this ion. For 10 points, name this divalent cation that is stored in bones.
Science

A mostly fabricated discussion of a rainy burial in this work follows a scene in which one character unsettles the protagonist with her burning eyes, which that protagonist describes as feral, in a darkened room he had come to looking to assault a man after embarrassingly arriving an hour early to a dinner. That man, an inheritor who the protagonist criticizes for peasant girl-related debauchery, mirrors a military man who earlier prompts the main character to purchase a beaver collar, for which he borrows money from Anton, his boss. The narrator is obsessed with the phrase two and two is four and clashes with Ferfichkin and Simonov over a going-away party for Zverkov, which prompts a trip to a brothel and later an exchange with the vulture-like Apollon. The kindly prostitute Liza is ultimately spurned by, FTP, the protagonist of what early existentialist novel consisting of the titular correspondences, written by Fyodor Dostoevsky
Literature

The propensity of nitroso compounds to participate in a variant of this reaction has been used to synthesize oxazine derivatives. Another variant of this reaction yields 2,3-dihydro-4-pyridones and uses a compound named for Danishefsky. In addition to that 'aza' variation, this reaction is usually thermodynamically favorable since it sees the formation two sigma bonds from pi bonds. Under kinetic control, it preferentially yields an endo product, and one component must be in the s-cis conformation for the reaction to occur. For 10 points, name this cycloaddition reaction in which a diene reacts with a dienophile to produce a ring and is named after two German chemists.

Science