Let A={1,2,3} ,B={1,2,3,4} and C={1,2,3,4,5,6}
from the above examples we see that all element in A belongs to B and all elements in B belong to or is a subset of C.
Since all elements in a belong to B so all elements of A will also belong to C because B is subset of C.
Hence Ais also a subset of C
