34779
Natural calcium is a mixture of five stable isotopes ( Ca , Ca , Ca , Ca , and Ca ) and one isotope with a half-life so long that it can be considered stable for all practical purposes ( Ca , with a half-life of about 4.3 " × " 10 " years ) .
 Ca, Ca
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Ca, Ca