How do the nitrogen bases pairs join in dna?

There are 4 nitrogenous bases in DNA. Adenine pairs with thymine, and Guanine pairs with cytosine. A Dna Nucleotide consists of a 5 carbon sugar, a phosphate group and a nitrogenous base. These bases are paired up by weak hydrogen bonds, so when they have have to replicate, the enzyme helicase can easily unzip the Dna. 

Idk i think the answer choice is number 1

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