Final answer:
To prove that if A is a subset of B and B is a subset of C, then A is a subset of C, we need to show that every element in A is also an element in C.
Step-by-step explanation:
To prove that if A is a subset of B and B is a subset of C, then A is a subset of C, we need to show that every element in A is also an element in C. Suppose x is an element of A. Since A is a subset of B, x is also an element of B. And since B is a subset of C, x is also an element of C. Therefore, every element in A is also an element in C, and we can conclude that A is a subset of C.