In which type of chemical reaction do two or more reactants combine to form one product, only?
a. synthesis
b. decomposition
c. single replacement
d. double replacement.

Synthesis is when multiple reactants come together to form one. Decomposition is when one reactant breaks down to form multiple products which are either elements or compounds. Single replacement is when there is an element and a compound and the element switch spots with the other element( metal switch with metal, nonmental with nonmetal, vice versa). The element has to be more reactive than the like element in the compound in order for it to be able to switch. Use the reactivity series to help you with this. Double Replacement is when you have two compounds and like elements switch places.

Do you mean semi-aqueous gypsum calcium?
Is called a semi-aqueous gypsum calcium sulfate, as the formula: CaSO4 * 1/2 H 2 O molecule of water is half (hemihydrate), contains sulfur (sulfate), and calcium (Ca).
Gypsum is a common name because it is used in construction.