Detailed Instructions: In this task, you are given a multiple-choice question about healthcare. Answer the question based on your information and classify your answers into '1', '2', '3', and '4'.
Q: Question: The DNA link number:
Options: <1> It is the number of hydrogen bonds of a double-stranded DNA. <2> It is the number of optimal matings in a double-stranded DNA. <3> It is the number of phosphoester bonds in a single-stranded DNA. <4> It is a topological property and defines the degree of supercoiling of a DNA. <5> It is the number of turns in double-stranded DNA.
A:
4