Task: Question Answering
Input (English): 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.
Output (English): 4