Task: Question Answering
Input (English): Question: The most commonly used data to study the phylogenetic relationship between bacteria is: 
 Options:  <1> The cytosine-guanine composition of their DNA. <2> The degree of DNA-DNA hybridization. <3> The amino acid sequence of ribosomal proteins. <4> The nucleotide sequence of 16S ribosomal RNA. <5> The composition of the cell wall.
Output (English): 4