Which one of the following discoveries involving dna happened first

The discoverers of DNA were not James D Watson and Francis Crick. They are the first scientist to formulate an accurate description of the DNA’s (deoxyribonucleic acid) complex , double-helical structure. They discovered that base pairing nucleotides must be; adenine and thymine; and cytosine and guanine. These base pairs are held together by a hydrogen bond, atype of chemical reaction that s easy to break and easy to reform. The statement is “DNA is a double helix made of two strands linked together with hydrogen bonds.”