All Acronyms. CTA - Control Terminal Agency [Internet]; April 13, 2017 [cited 2017 APR 13]. Available from: https://www.allacronyms.com/CTA/Control_Terminal_Agency.
There are almost 120 known elements. For example, you are made up of billions of billions of atoms but you probably won't find more than 40 elements (types of atoms) in your body. Chemists have learned that over 95% of your body is made up of hydrogen (H), carbon (C), nitrogen, oxygen, phosphorus (P), and calcium (Ca).
Are they 12 distinct elements? then subsets of S with at most 3 elements must have no elements, 1 element, 2 elements or 3 elements. How many subsets are there with 1 element? Because from 12 elements you are selecting 1 element.