Task: Question Answering
Input (English): Question: The color centers or F centers are produced: 
 Options:  <1> Exclusively in crystals that contain fluorine. <2> By heating a crystal in the vapor of an alkali metal. <3> By cooling a crystal in the vapor of an alkali metal. <4> By heating a crystal in the presence of fluorine. <5> By cooling a crystal in the presence of fluorine.
Output (English): 2