(b) : The two $DNA$ chains are held together by hydrogen bonds between their nitrogenous bases. Adenine $(A)$, a purine of one chain lies exactly opposite thymine $(T)$, a pyrimidine of the other chain. Similarly, cytosine $(C)$, a pyrimidine lies opposite guanine $(G)$, a purine. Three hydrogen bonds occur between cytosine and guanine $(C \cong G)$ at positions $1', 2'$, and $6'$ and two hydrogen bonds between adenine and thymine $(A = T)$ at positions $1'$ and $6'$.