Task: Question Answering
Input (English): Question: Calcium fluoride is a white solid that is found as such in nature (fluorspar or fluorite, which can occur in many colors) and is used as an important source of fluoride. Among its physical and chemical properties include: 
 Options:  <1> Its solubility in water, which differentiates it from the other group 2 fluorides. <2> Its resistance to concentrated sulfuric acid. <3> Its hydrolysis in water, which transforms it into CaO and HF. <4> Its relatively low melting point for a mineral. <5> Its favorable, although slow, oxidation to the air.
Output (English): 4